Bug#564448: xserver-xorg-video-radeon: black screen when switching from VT to X, or waking up from sleep
I had this same problem. I have the exact same card. However, I never tried 6.12.1, so I don't know if it was working then.
I was able to find a fix for the problem, which is rather strange. Are you running gnome? if so, in a terminal type gconf-editor. Now browse to /apps/gnome-power-manager/general. On your right you should be able to see several check boxes. Please enable network_sleep. If you are not running gnome, there should be a way to activate this KDE, provided you are using NetworkManager.
After this, try suspending/hibernating and post back results.
It works well for me with 6.12.6 in squeeze and User modesetting.
Good luck!
Andres
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