I run a wheezy but upgraded the xserver-xorg-video-nouveau package to 1.0.1-2 (amd64) using the unstable repository, in the hope of solving it might fix issue.
When desktop compositing effects are on, moving or resizing windows will very often (but unpredictably) freeze the whole system. No keyboard, no mouse, no ping, nothing. I have to hard reboot. I don't have any issues when running without the desktop compositing effects. It's reasonable to assume the problem is related to acceleration. The graphic card is an NVS Quadro 290. I didn't have this problem last month, I suspect the problem was introduced with this upgrade http://snapshot.debian.org/package/xserver-xorg-video-nouveau/1%3A0.0.16%2Bgit20120614%2B36d3f8c-1/
