there's a bug submitted to the Gentoo buglist as well, with the same content:

http://bugs.gentoo.org/show_bug.cgi?id=52749

playing around with setxkbmap, and the documentation found on the XFree86 site
at http://www.xfree.org/4.3.0/XKB-Config2.html#2 , it seems that the heart of
the problem is that there is no file named hu in the
/usr/X11R6/lib/X11/xkb/symbols/pc directory, even though there should be.

$ setxkbmap -layout "us" -print
xkb_keymap {
        xkb_keycodes  { include "xfree86+aliases(qwerty)"       };
        xkb_types     { include "complete"      };
        xkb_compat    { include "complete"      };
        xkb_symbols   { include
"pc/pc(pc105)+pc/us+inet(presario)+group(ctrl_shift_toggle)+group(alt_shift_toggle)"
   };
        xkb_geometry  { include "pc(pc104)"     };
};

$ setxkbmap -layout "us" -print | xkbcomp -
Warning:          No symbols defined for <SYRQ> (keycode 92)
Warning:          No symbols defined for <BRK> (keycode 114)
(... more such warnings here)

$ setxkbmap -layout "hu" -print
xkb_keymap {
        xkb_keycodes  { include "xfree86+aliases(qwerty)"       };
        xkb_types     { include "complete"      };
        xkb_compat    { include "complete"      };
        xkb_symbols   { include
"en_US(pc105)+hu+inet(presario)+group(ctrl_shift_toggle)+group(alt_shift_toggle)"
      };
        xkb_geometry  { include "pc(pc104)"     };
};

$ setxkbmap -layout "hu" -print | xkbcomp -
Warning:          No symbols defined for <SYRQ> (keycode 92)
Warning:          No symbols defined for <BRK> (keycode 114)
(... more such warnings here)

$ setxkbmap -layout "us,hu" -print
xkb_keymap {
        xkb_keycodes  { include "xfree86+aliases(qwerty)"       };
        xkb_types     { include "complete"      };
        xkb_compat    { include "complete"      };
        xkb_symbols   { include
"pc/pc(pc105)+pc/us+inet(presario)+pc/hu:2+group(ctrl_shift_toggle)+group(alt_shift_toggle)"
   };
        xkb_geometry  { include "pc(pc104)"     };
};

see the reference above for pc/hu...

$ setxkbmap -layout "us,hu" -print | xkbcomp -
Error:            Can't find file "pc/hu" for symbols include
                  Exiting
                  Abandoning symbols file "(null)"

and here is the real problem...

now copying say the file de in /usr/X11R6/lib/X11/xkb/symbols/pc to hu makes the
above pass, but I still can't add a hungarian layout through the gnome keyboard
preferences :(

Unfortunately, Hungarian layouts are problematic in all recent X servers. And
gnome has nothing to do with it. I'd say the problem should be reported to the
respective X servers' bugzillas.

well, I'm not entirely sure if this is the case. until recently, one could
change keyboard layouts with a small applet, gkb was the name as I recall it:
http://projects.gnome.hu/gkb/
It used xmodmap and other commands to change the layout. One could change to
hungarian layouts without any problems. No X server objected to it, it was
working fine. (and it would have been very strange, as one of the authors of gkb
is a hungarian)

Now I can not find this applet in my gnome anymore, and I can't change to
hungarian layout at all. This is a _very_ serious problem: one can not write
_any_ hungarian text on gnome at the moment. basically, this makes it totally
useless for any hungarian user.

as for your above suggestion, I have filed a bugzilla entry in the XFree86 bugzilla:

http://bugs.xfree86.org/show_bug.cgi?id=1461

the following bug report details the same bug under Xorg:

http://freedesktop.org/bugzilla/show_bug.cgi?id=1164

I have fixed gkb applet in the cvs but it's freeze (what I haven't recognised in time). If everything goes 
well gkb will be reachable under Gnome again. 

I got complaints from Sun people, too (they haven't got xkb).

diff from the cvs (sorry for the format, just a fast cvs diff) and the new file will be attached soon 

Created attachment 31520 [details]
GKB .server file (needs some 'administration' in Makefile.am and some new code in gkb-applet.c)

This is the missing file, that's why GKB is not in the applet adding dialog

Created attachment 31521 [details] [review]
cvs diff from yesterday

* gkb-applet.c: Factory code for woring with applet chooser
* Makefile.am: Executable from shlib, .server file and cleanup
* gkb.c: little cosmetics for flag mode, default theme is label mode, I want to
make PC applet
* gkb.schemas.in: fixing schemas, too...

Shooby: in your attachement, you effectively killed libgkb.a - so it is not
possible for the Keyboard Indicator to become "gkb-new" in XKB-less
environments. Do you think it is good? Could we get together on IRC with jody
and kmaaras to discuss this for 2.10?

Please use 'diff -u' for any patches in the future. That is the de facto
standard for patches and in many people's opinion much easier to read.

Not only Hungarian,

Today I wanted to add Azeri keyboard but i coudln't.
Error:
###########################################
# Error activating XKB configuration.     #
# Probably internal X server problem.     #
#                                         #
#  X server version data:                 #
# The X.Org Foundation                    #
# 60700000                                #
###########################################
I tested other layouts and the result is:
(My GNOME is 2.6 in slackware 10)
All of these layouts have this problem:

Bosnian
Azerbaijani
Canadian
Faroese
French comb...
Hungarian
Japanease
Korean
Lithuanian azerty
Macdonian
Mongolian
PC-98xx series
Romanian
Swiss French
Swiss Germany
Vietnamese

I am having the same problem with Gnome 2.6.2 on Debian SID. There were a bunch
of bug reports similar to this in Redhat bugzilla but none of them helped. I
currently use "setxkbmap az" command to switch to Azeri layout which just works,
and even though I have removed the applet from panel, I get the above stated
error when Gnome-session is loading.

What is the status of this?

it still doesn't work, with my current Keyboard Indicator version, which is 2.8.0

I know xmodmap work went into G9. Sergey, have you addressed this issue?

Well, there is a mix of bugs here. First, Hungarian is really broken in all
released servers (including latest Xorg and XFree). It is only fixed in
xkeyboard-config. So I consider the case for 'hu' as NONGNOME. Full stop.

Now, regarding the 'az' layout. If "setxkbmap" works with it ok - so should
GNOME. Metin, did you try the latest 2.9.x series?

I'm sorry, but I have to disaggree with you on the hungarian layout issue -
setxkbmap works perfectly for hungarian with xorg-x11 and with xfree86 as well.

actually, since tha gnome applet is broken, this is how I switch between english
and hungarian layouts - by issuing setxkbmap us and setxkbmap hu, respectively.

>Metin, did you try the latest 2.9.x series?

Sereja, I am running Ubuntu Hoary and unfortunately the problem is still there.
I too use setxkbmap us and setxkbmap az, defined them in gconf-editor so that I
could add some keyboard shortcuts to them. Thanks.

Akos: Hunguarian is broken in respect that it cannot be used together with out
layouts. Could you try please - setxkbmap us,hu - and see what happens?

Metin: could you please try "setxkbmap us,az"? Will it give any errors/warnings?

you're right, it does give errors:

$ setxkbmap us,hu
Error loading new keyboard description

Akos: that is why I close discussins on Hungarian layout:) Actually, this should
be in FAQs... Same about Canadian and Japanese things...

From the comments, this would appear to be NOTGNOME then.