We tend to encounter major classes of bugs when we upload to unstable. This is a place to track what's currently a problem there.
Firmware files for ATI gfxcards
Make sure you have installed the latest firmware-linux package:
# apt-get update && apt-get install firmware-linux
Check loading of firmware via:
$ dmesg | egrep -i 'radeon|firmware'
Some R600/R700 gfxcards need additional firmware files starting with kernel 2.6.33: R{600,700}_rlc.bin (download files from [2])
[1] http://wiki.x.org/wiki/radeonBuildHowTo -> Troubleshooting Extra Firmware for R600/R700
[2] http://people.freedesktop.org/~agd5f/radeon_ucode/
Broken xserver-xorg-video-nv
Due to 564203 the nv video driver in unstable crashes with xserver 1.7.
Some users report success with self-debianized versions of latest upstream xf86-video-nv-2.1.16 built against xorg-server (1.7.3.902-1). Others claim colour corruptions on some Nvidia gfxcards with 2.1.16.