Hmm - bad news once again:

I have now compiled and tested a 3.6.kernel to match the commit immediately
before the "mega commit", i.e. the kernel tarball has been produced by the
following command:

$ git archive --format=tar "ebb945a94bba2ce8dff7b0942ff2b3f2a52a0a69^" | bzip2
> ~/Projekte/nouveau_drm/linux-before-mega.tar.bz2

Unfortunately, I am unable to test whether the screen distortion issue occurs
with this kernel, because I get a complete hang (system freezes, CPU and GPU
fans running full speed) somewhere between some seconds and some minutes after
starting GNOME...

Note that I have seen both: either no screen corruption at all or first slight
signs of screen corruption (white rectangles around window frames) at the times
of the hangs.

The error messages that I find in /var/log/messages probably associated with
the hangs (sorry, I can't get any messages ot of dmesg due to the hang...) seem
to be the following:

[drm:drm_mm_takedown] *ERROR* Memory manager not clean. Delaying takedown
[drm:drm_mm_takedown] *ERROR* Memory manager not clean. Delaying takedown
[drm:drm_mm_takedown] *ERROR* Memory manager not clean. Delaying takedown

repeating any number between 3 to 5 times directly before the hangs
(immediately followed by /var/log/messages starting over with my power-off
machine restart).

Will now move forward to test with the most recent stock kernel from the 3.6
series: 3.6.11-3.6.y.20121225.ol6 from the Oracle public yum playground to see
whether this already is affected... :-(

BR,
Andreas