When MusicBrainz switched to the new Next Generation Schema, it now supports multiple media (i.e. CDs) per release.  Unfortunately this change appears to mess up rhythmbox's support for reading multiple discs.

In this example, I'm using rhythmbox-0.13.3-1.fc14.x86_64 on Fedora 14.  What happens is when I insert disc 2, it still returns the tracklist for disc 1 for this Orbital release:

http://musicbrainz.org/release/dab39a9e-44ca-34ff-ade6-1fb8424ee3a1

This release happens to have 8 tracks on each disc, but both CDs only return the tracklist for disc 1, even though the discIDs are different.  I reported this to MusicBrainz and they said they suspect the problem is that rhythmbox is requesting the tracklist of the release after identifying which release the disc belongs to (apparently people have reported the same thing happening in other cd ripping programs).  It is reported on upstream MusicBrainz here:

http://tickets.musicbrainz.org/browse/MBS-2407

Here is the console output when looking up disc 2 in the Orbital release mentioned above:

MusicBrainz: Connecting to http://musicbrainz.org:80
MusicBrainz: GET /ws/1/release/?type=xml&discid=GbYEXM0NckYsBOd7imJ0vwRCQDY-
MusicBrainz: Result: 0 (200 )
MusicBrainz: Status: 200
MusicBrainz: Response:
<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-1.0#" xmlns:ext="http://musicbrainz.org/ns/ext-1.0#"><release-list><release type="Live Official" id="672509dd-1971-4a4d-b29b-e6cdbd1d8267" ext:score="100"><title>Orbital: Live at Glastonbury 1994-2004</title><text-representation script="Latn" language="ENG" /><asin>B000PTYND6</asin><artist id="f3e2a7d9-c6bb-4848-95e5-04c0a1e2f511"><name>Orbital</name><sort-name>Orbital</sort-name></artist><track-list><track id="edcd40f7-fb23-4018-82d9-a4ee37ac0a2f"><title>Style / Bagpipe Style (1999)</title><duration>537200</duration></track><track id="b9e26bd6-a653-4dd0-a82a-89f0cd13bacd"><title>The Girl With the Sun in Her Head (2002)</title><duration>358040</duration></track><track id="2b9c6e45-1a61-4801-aeb2-355fa3d918eb"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586800</duration></track><track id="4743ebb9-177a-424d-ab7c-ff205482d179"><title>Belfast (2002)</title><duration>450466</duration></track><track id="f9a503cd-43fa-4651-b279-4927ae691011"><title>Frenetic (2002)</title><duration>545333</duration></track><track id="332ea346-566b-4f62-a3fc-3dd3248fde1e"><title>Satan (2004)</title><duration>420866</duration></track><track id="b976e9af-48f3-41f1-89cf-3ccac619d69b"><title>Dr Who? (2004)</title><duration>326893</duration></track><track id="f7ad1f96-9192-4a64-8d60-3355afbd409b"><title>Chime (2004)</title><duration>678106</duration></track></track-list><release-event-list><event country="GB" date="2007-06-11" barcode="5390537970484" /></release-event-list><disc-list count="1" /></release><release type="Live Official" id="dab39a9e-44ca-34ff-ade6-1fb8424ee3a1" ext:score="100"><title>Orbital: Live at Glastonbury 1994-2004</title><text-representation script="Latn" language="ENG" /><asin>B000Y80L3O</asin><artist id="f3e2a7d9-c6bb-4848-95e5-04c0a1e2f511"><name>Orbital</name><sort-name>Orbital</sort-name></artist><track-list><track id="edcd40f7-fb23-4018-82d9-a4ee37ac0a2f"><title>Style / Bagpipe Style (1999)</title><duration>537200</duration></track><track id="b9e26bd6-a653-4dd0-a82a-89f0cd13bacd"><title>The Girl With the Sun in Her Head (2002)</title><duration>358040</duration></track><track id="2b9c6e45-1a61-4801-aeb2-355fa3d918eb"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586800</duration></track><track id="4743ebb9-177a-424d-ab7c-ff205482d179"><title>Belfast (2002)</title><duration>450466</duration></track><track id="f9a503cd-43fa-4651-b279-4927ae691011"><title>Frenetic (2002)</title><duration>545333</duration></track><track id="332ea346-566b-4f62-a3fc-3dd3248fde1e"><title>Satan (2004)</title><duration>420866</duration></track><track id="b976e9af-48f3-41f1-89cf-3ccac619d69b"><title>Dr Who? (2004)</title><duration>326893</duration></track><track id="f7ad1f96-9192-4a64-8d60-3355afbd409b"><title>Chime (2004)</title><duration>678106</duration></track></track-list><release-event-list><event country="US" date="2007" barcode="880882159221" /></release-event-list><disc-list count="1" /></release></release-list></metadata>
MusicBrainz: Connecting to http://musicbrainz.org:80
MusicBrainz: GET /ws/1/release/672509dd-1971-4a4d-b29b-e6cdbd1d8267?type=xml&inc=artist%20tracks%20release-events%20url-rels
MusicBrainz: Result: 0 (200 )
MusicBrainz: Status: 200
MusicBrainz: Response:
<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-1.0#" ><release type="Live Official" id="672509dd-1971-4a4d-b29b-e6cdbd1d8267"><title>Orbital: Live at Glastonbury 1994-2004</title><text-representation script="Latn" language="ENG" /><asin>B000PTYND6</asin><artist id="f3e2a7d9-c6bb-4848-95e5-04c0a1e2f511"><name>Orbital</name><sort-name>Orbital</sort-name></artist><track-list><track id="ec96fc47-b32d-4f19-b6fd-c6bbdc2057cc"><title>Walk Now (1994)</title><duration>482973</duration></track><track id="46fab439-82fb-44ef-a328-0f43981772e7"><title>Are We Here? (1994)</title><duration>944066</duration></track><track id="c3b75410-65ac-4d5b-968d-9868afd94a75"><title>Attached (1994)</title><duration>501426</duration></track><track id="5971c30a-2674-4866-abe5-f2921b30a8cc"><title>Kein Trink Wasser (1995)</title><duration>458333</duration></track><track id="4af7a1a8-8e9c-4b9c-9d04-0c665b0dec38"><title>Impact (The Earth Is Burning) (1995)</title><duration>635466</duration></track><track id="35eb2ade-acb6-443b-8cc3-3f134819042d"><title>Remind (1995)</title><duration>559106</duration></track><track id="03b8d9a6-3b3c-4fb2-9f12-7afac4df4716"><title>Halcyon (1999)</title><duration>484693</duration></track><track id="a3c3a11a-9a49-44ee-a905-dae584d2e301"><title>The Box (1999)</title><duration>320706</duration></track><track id="edcd40f7-fb23-4018-82d9-a4ee37ac0a2f"><title>Style / Bagpipe Style (1999)</title><duration>537200</duration></track><track id="b9e26bd6-a653-4dd0-a82a-89f0cd13bacd"><title>The Girl With the Sun in Her Head (2002)</title><duration>358040</duration></track><track id="2b9c6e45-1a61-4801-aeb2-355fa3d918eb"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586800</duration></track><track id="4743ebb9-177a-424d-ab7c-ff205482d179"><title>Belfast (2002)</title><duration>450466</duration></track><track id="f9a503cd-43fa-4651-b279-4927ae691011"><title>Frenetic (2002)</title><duration>545333</duration></track><track id="332ea346-566b-4f62-a3fc-3dd3248fde1e"><title>Satan (2004)</title><duration>420866</duration></track><track id="b976e9af-48f3-41f1-89cf-3ccac619d69b"><title>Dr Who? (2004)</title><duration>326893</duration></track><track id="f7ad1f96-9192-4a64-8d60-3355afbd409b"><title>Chime (2004)</title><duration>678106</duration></track><track id="811f5266-c5a9-4d46-94ea-3166162d6291"><title>Walk Now (1994)</title><duration>482000</duration></track><track id="3aae35ca-8699-44ec-bdd4-e8ce0b8069cf"><title>Are We Here? (1994)</title><duration>944000</duration></track><track id="43a21b69-2d41-49a0-9456-9c49dd27cb49"><title>Attached (1994)</title><duration>501000</duration></track><track id="b887b4eb-a4d6-4462-94a6-76f8212e015f"><title>Kein Trink Wasser (1995)</title><duration>458000</duration></track><track id="d74bd23d-60c2-4c89-a72c-d56e42e39645"><title>Impact (The Earth Is Burning) (1995)</title><duration>635000</duration></track><track id="7d3d4544-7775-4ed8-9a8b-69b813068bb9"><title>Remind (1995)</title><duration>559000</duration></track><track id="f22e8836-1325-42bf-bd09-6e3bcec73b3a"><title>Halcyon (1999)</title><duration>484000</duration></track><track id="d66c160a-609c-4cdc-a794-fa3f6adff6f7"><title>The Box (1999)</title><duration>320000</duration></track><track id="cc61d4f0-179c-4d8f-af12-3b5d3e4abce1"><title>Style / Bagpipe Style (1999)</title><duration>537000</duration></track><track id="559c77c6-f94c-4701-92bf-3799221ba5f3"><title>The Girl With The Sun In Her Head (2002)</title><duration>358000</duration></track><track id="dd4875f5-6f45-4b04-b85e-56789f5926a3"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586000</duration></track><track id="fb70c3e2-aaee-4750-a32e-0c624bad5cb7"><title>Belfast (2002)</title><duration>450000</duration></track><track id="ba68a5ce-dd62-4e78-816c-cb8e6b9706f5"><title>Frenetic (2002)</title><duration>545000</duration></track><track id="66730231-2be3-43ee-b23d-1a194b9fee7e"><title>Satan (2004)</title><duration>420000</duration></track><track id="daf022c4-0d1a-4490-832d-16120fdbcfdb"><title>Dr Who? (2004)</title><duration>326000</duration></track><track id="33111bd9-f041-412e-9374-32d04587fe12"><title>Chime (2004)</title><duration>678000</duration></track></track-list><release-event-list><event country="GB" format="2xCD + DVD-Video" date="2007-06-11" barcode="5390537970484" catalog-number="ACP002" /></release-event-list><relation-list target-type="Url"><relation target="http://www.amazon.co.uk/gp/product/B000PTYND6" type="AmazonAsin" /><relation target="http://www.discogs.com/release/992927" type="Discogs" /></relation-list></release></metadata>
MusicBrainz: Connecting to http://musicbrainz.org:80
MusicBrainz: GET /ws/1/release/dab39a9e-44ca-34ff-ade6-1fb8424ee3a1?type=xml&inc=artist%20tracks%20release-events%20url-rels
MusicBrainz: Result: 0 (200 )
MusicBrainz: Status: 200
MusicBrainz: Response:
<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-1.0#" ><release type="Live Official" id="dab39a9e-44ca-34ff-ade6-1fb8424ee3a1"><title>Orbital: Live at Glastonbury 1994-2004</title><text-representation script="Latn" language="ENG" /><asin>B000Y80L3O</asin><artist id="f3e2a7d9-c6bb-4848-95e5-04c0a1e2f511"><name>Orbital</name><sort-name>Orbital</sort-name></artist><track-list><track id="ec96fc47-b32d-4f19-b6fd-c6bbdc2057cc"><title>Walk Now (1994)</title><duration>482973</duration></track><track id="46fab439-82fb-44ef-a328-0f43981772e7"><title>Are We Here? (1994)</title><duration>944066</duration></track><track id="c3b75410-65ac-4d5b-968d-9868afd94a75"><title>Attached (1994)</title><duration>501426</duration></track><track id="5971c30a-2674-4866-abe5-f2921b30a8cc"><title>Kein Trink Wasser (1995)</title><duration>458333</duration></track><track id="4af7a1a8-8e9c-4b9c-9d04-0c665b0dec38"><title>Impact (The Earth Is Burning) (1995)</title><duration>635466</duration></track><track id="35eb2ade-acb6-443b-8cc3-3f134819042d"><title>Remind (1995)</title><duration>559106</duration></track><track id="03b8d9a6-3b3c-4fb2-9f12-7afac4df4716"><title>Halcyon (1999)</title><duration>484693</duration></track><track id="a3c3a11a-9a49-44ee-a905-dae584d2e301"><title>The Box (1999)</title><duration>320706</duration></track><track id="edcd40f7-fb23-4018-82d9-a4ee37ac0a2f"><title>Style / Bagpipe Style (1999)</title><duration>537200</duration></track><track id="b9e26bd6-a653-4dd0-a82a-89f0cd13bacd"><title>The Girl With the Sun in Her Head (2002)</title><duration>358040</duration></track><track id="2b9c6e45-1a61-4801-aeb2-355fa3d918eb"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586800</duration></track><track id="4743ebb9-177a-424d-ab7c-ff205482d179"><title>Belfast (2002)</title><duration>450466</duration></track><track id="f9a503cd-43fa-4651-b279-4927ae691011"><title>Frenetic (2002)</title><duration>545333</duration></track><track id="332ea346-566b-4f62-a3fc-3dd3248fde1e"><title>Satan (2004)</title><duration>420866</duration></track><track id="b976e9af-48f3-41f1-89cf-3ccac619d69b"><title>Dr Who? (2004)</title><duration>326893</duration></track><track id="f7ad1f96-9192-4a64-8d60-3355afbd409b"><title>Chime (2004)</title><duration>678106</duration></track><track id="811f5266-c5a9-4d46-94ea-3166162d6291"><title>Walk Now (1994)</title><duration>482000</duration></track><track id="3aae35ca-8699-44ec-bdd4-e8ce0b8069cf"><title>Are We Here? (1994)</title><duration>944000</duration></track><track id="43a21b69-2d41-49a0-9456-9c49dd27cb49"><title>Attached (1994)</title><duration>501000</duration></track><track id="b887b4eb-a4d6-4462-94a6-76f8212e015f"><title>Kein Trink Wasser (1995)</title><duration>458000</duration></track><track id="d74bd23d-60c2-4c89-a72c-d56e42e39645"><title>Impact (The Earth Is Burning) (1995)</title><duration>635000</duration></track><track id="7d3d4544-7775-4ed8-9a8b-69b813068bb9"><title>Remind (1995)</title><duration>559000</duration></track><track id="f22e8836-1325-42bf-bd09-6e3bcec73b3a"><title>Halcyon (1999)</title><duration>484000</duration></track><track id="d66c160a-609c-4cdc-a794-fa3f6adff6f7"><title>The Box (1999)</title><duration>320000</duration></track><track id="cc61d4f0-179c-4d8f-af12-3b5d3e4abce1"><title>Style / Bagpipe Style (1999)</title><duration>537000</duration></track><track id="559c77c6-f94c-4701-92bf-3799221ba5f3"><title>The Girl With The Sun In Her Head (2002)</title><duration>358000</duration></track><track id="dd4875f5-6f45-4b04-b85e-56789f5926a3"><title>Funny Break (Weekend Ravers) (2002)</title><duration>586000</duration></track><track id="fb70c3e2-aaee-4750-a32e-0c624bad5cb7"><title>Belfast (2002)</title><duration>450000</duration></track><track id="ba68a5ce-dd62-4e78-816c-cb8e6b9706f5"><title>Frenetic (2002)</title><duration>545000</duration></track><track id="66730231-2be3-43ee-b23d-1a194b9fee7e"><title>Satan (2004)</title><duration>420000</duration></track><track id="daf022c4-0d1a-4490-832d-16120fdbcfdb"><title>Dr Who? (2004)</title><duration>326000</duration></track><track id="33111bd9-f041-412e-9374-32d04587fe12"><title>Chime (2004)</title><duration>678000</duration></track></track-list><release-event-list><event country="US" format="2xCD + DVD-Video" date="2007" barcode="880882159221" catalog-number="ATO0041" /></release-event-list><relation-list target-type="Url"><relation target="http://www.amazon.com/gp/product/B000Y80L3O" type="AmazonAsin" /><relation target="http://www.discogs.com/release/1348099" type="Discogs" /></relation-list></release></metadata>