Sorry, but i have to REOPEN the bug again. It seems my cheers came to rashly. The freezes dont happen that often anymore, but they STILL happen, leading to a complete freeze of the system.
Steps to reproduce:
Watch a movie with VLC. At some random intervall the machine just freezes.
Expected behaviour: It should not crash

Indicator that this is a bug:
Im not 100% sure, that this is a bug in the kernel itself, since it may also be related to Xorg or the radeon driver. But since i can prevent the system from freezing by passing "modeset.radeon=0" to the kernel at boottime i strongly assume its a kernel specific bug.
I tried several
kernel versions from 2.6.32 to the actual wheezy kernel. The bug is still present. I will also try the experimental one, but it seems obvious, that they didnt improve the code, which causes this bug, after so many versions.
I added the dmesg output from my system. I got it right after the freeze, booting to Knoppix and copying the whole /var/log folder. So if you need other logs as well, please say so.
cheers