Now that we have DRM from 2.6.33 in latest 2.6.32 kernel in unstable, I just uploaded Radeon KMS and DRI2 to unstable. xserver-xorg-video-radeon 1:6.12.192-2 even enables KMS by default. Please test it.
In case of problems, you may for instance disable KMS by changing modeset to 0 in /etc/modprobe.d/radeon-kms.conf. You may also downgrade to testing where xserver-xorg-video-radeon 1:6.12.6-1 does not enable/support KMS.
Make sure you run linux-image-2.6.32-4-$arch or later so that you actually have DRM from 2.6.33 and the radeon kernel module gets loaded early by udev. Otherwise, you may experience problems like this. You may need to add radeon to /etc/modules as a temporary fix.