Is this still a problem with the newer radeon driver in sid (also newer kernel and mesa)? (make sure you have libdrm-radeon1, firmware-linux-nonfree and firmware-linux installed). If problem persists, could you try getting newer 2.6.33 from experimental. When enabling Kernel modesetting, add * radeon* to /etc/modules and put *options radeon modeset=1* in /etc/modprobe.d/radeon-kms.conf (you might need to create the file).
Report back! Andres
