Description of problem:
I attempted to enable compiz using System -> Preferences -> Desktop Effects. X crashed instantly, putting me back to gdm login screen.

Version-Release number of selected component (if applicable):
xorg-x11-drv-ati-6.12.2-6.fc11.x86_64
xorg-x11-server-Xorg-1.6.1-6.fc11.x86_64
kernel-2.6.29.1-104.fc11.x86_64

How reproducible:
almost always

Steps to Reproduce:
1. Run desktop-effects
2. Push the button to enable the effects.
3. If by sheer luck you get to step 3, push the button again to disable the effects.
  
Actual results:
X will likely crash after step 2. If it does not, then it will almost certainly crash after step 3.

Expected results:
X should survive, with compiz running after step 2.

Additional info:
The hardware is ATI Technologies Inc RS690M [Radeon X1200 Series] [1002:791f] in a Toshiba Satellite Pro laptop.

Backtrace:
0: /usr/bin/Xorg(xorg_backtrace+0x26) [0x4e9d26]
1: /usr/bin/Xorg(xf86SigHandler+0x6f) [0x47de1f]
2: /lib64/libc.so.6 [0x3a88633570]
3: /usr/lib64/dri/r300_dri.so(radeonDestroyBuffer+0x40) [0x7f9ecce9aafb]
4: /usr/lib64/dri/r300_dri.so(radeonDestroyContext+0xdf) [0x7f9eccebd07e]
5: /usr/lib64/dri/r300_dri.so [0x7f9ecce94d7c]
6: /usr/lib64/xorg/modules/extensions//libglx.so [0x7f9ece630c99]
7: /usr/lib64/xorg/modules/extensions//libglx.so(__glXFreeContext+0x6c) [0x7f9ece62683c]
8: /usr/lib64/xorg/modules/extensions//libglx.so [0x7f9ece626bcb]
9: /usr/bin/Xorg(CallCallbacks+0x4c) [0x44b6ec]
10: /usr/bin/Xorg(CloseDownClient+0x58) [0x441ec8]
11: /usr/bin/Xorg(Dispatch+0x210) [0x447040]
12: /usr/bin/Xorg(main+0x3c5) [0x42d0e5]
13: /lib64/libc.so.6(__libc_start_main+0xfd) [0x3a8861e83d]
14: /usr/bin/Xorg [0x42c559]

Fatal server error:
Caught signal 11.  Server aborting