This bug seems to be solved in Squeezy, but is present in Wheezy. I just took the Wheezy Beta 1 CD and my screen went right away into "white noise" state:
http://www.shcherbyna.com/?p=1906 (I tried both GNOME & KDE editions).
I had to install firmware-linux-nonfree via ssh and it solved the problem. I've never experienced this problem on my machine in Squeezy and was surprised to see in Wheezy. Is it a regression?
Thanks,
--
with best regards, Volodymyr.