is this still a problem with the newer radeon driver from squeeze? If so, could you try getting the even newer one from sid (you'll need a newer kernel as well, either 2.6.32-10 from sid or 2.6.33 from experimental). For this you will need to enable Kernel modesetting. add *radeon* to /etc/modules and put *options radeon modeset=1* in /etc/modprobe.d/radeon-kms.conf (you'll probably need to create the file).
Andres
