This is still quite elusive. It only happens once every couple of days,
and the only thing that seems to be clear is that it's triggered by a GL
screensaver. So if it's ok I'll continue sharing X crash backtraces
while I go on groping for a lead. Here's my next one, this time with
kernel 2.6.25-rc8:

0: /usr/bin/X(xf86SigHandler+0x81) [0x80e6d81]
1: [0xb7f22400]
2:
/usr/lib/xorg/modules//extensions/libGLcore.so(_mesa_set_enable+0x1343)
[0xb50aa673]
3: /usr/lib/xorg/modules//extensions/libGLcore.so(_mesa_Disable+0x5a)
[0xb50abc1a]
4: /usr/lib/xorg/modules//extensions/libglx.so [0xb7ba8b98]
5: /usr/lib/xorg/modules//extensions/libglx.so(DoRender+0xdd) [0xb7ba145d=
]
6: /usr/lib/xorg/modules//extensions/libglx.so [0xb7ba157c]
7: /usr/lib/xorg/modules//extensions/libglx.so [0xb7ba5a6d]
8: /usr/bin/X [0x815809e]
9: /usr/bin/X(Dispatch+0x1af) [0x808f68f]
10: /usr/bin/X(main+0x47e) [0x807717e]
11: /lib/libc.so.6(__libc_start_main+0xe0) [0xb7cb6fe0]
12: /usr/bin/X(FontFileCompleteXLFD+0x1e5) [0x8076501]

As for Pavel's suggestion of disabling heap randomization, given the
current crash rate I'd have to try that for several weeks before I could
be reasonably sure that it helped. I think it'll be more useful if I go
on looking for a way to reproduce the crash more reliably. Perhaps I
should start playing some GL based games. :-)

Hope this helps (slim chance, I know)
T.