Downgrading xserver-xorg-video-ati to feisty's 1:6.6.3-2ubuntu6 fixes the problem for me, so the problem must be in the changes between that version and 1:6.6.193-1ubuntu1.
Downgrading xserver- xorg-video- ati to feisty's 1:6.6.3-2ubuntu6 fixes the problem for me, so the problem must be in the changes between that version and 1:6.6.193-1ubuntu1.