In case you didn't find the solution yet (or if someone else is looking for it), take a look
at Debian Bug Report #482992
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482992
In short the solution is to add the following line to the "Device" section of
/etc/X11/xorg.conf
Option "XaaNoOffscreenPixmaps" "on"
You need to restart your X, unless you want to experiment with this:
http://ubuntuforums.org/showthread.php?p=4211618