Bisected to the following:

368d75fe75d0043e0cf90dffdac71fc3b75c28a1 is the first bad commit
commit 368d75fe75d0043e0cf90dffdac71fc3b75c28a1
Author: Sebastian Dröge <sebastian@centricular.com>
Date:   Thu Jul 10 12:39:46 2014 +0200

    audiodecoder: Handle CAPS events immediately instead of delaying them
    
    https://bugzilla.gnome.org/show_bug.cgi?id=733147

:040000 040000 d6e8e97978bf30aa91d8bcc06a0c4c083c2acd2a c863947b2930e5619ff2b946b5dae36ad8c6491d M	gst-libs

Here is what I get with decodebin (playbin as well..):

(jhbuild)> gst-launch-1.0 filesrc location=a.mp3 ! decodebin ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
0:00:00.073037720 31701      0x137c280 ERROR                 mpg123 gstmpg123audiodec.c:513:gst_mpg123_audio_dec_set_format:<mpg123audiodec0> Allowed src caps are NULL
Missing element: MPEG-1 Layer 3 (MP3) decoder
ERROR: from element /GstPipeline:pipeline0/GstDecodeBin:decodebin0: Your GStreamer installation is missing a plug-in.
Additional debug info:
gstdecodebin2.c(4063): gst_decode_bin_expose (): /GstPipeline:pipeline0/GstDecodeBin:decodebin0:
no suitable plugins found
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...
(jhbuild)> 

On the other hand this works:

(jhbuild)> gst-launch-1.0 filesrc location=a.mp3 ! mpegaudioparse ! mpg123audiodec ! fakesink
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
Got EOS from element "pipeline0".
Execution ended after 0:00:00.503072607
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...