Not just ATI 6xxx
From the parallels kb:
Cause
This OS X issue occurs on ATI 6xxx video card on OS X Mountain Lion only.
That's what the KB says, but it is absolutely false.
If someone from the Parallels team needs more details, feel free to contact me.
I have a MBP mid-2010 (MacPro5,1) with the ATI Radeon HD 5770 card. It drives three monitors: a pre-Thunderbolt 27" LED Cinema Display (2560x1440), a Dell U2412M display (1920x1200) and a Samsung Syncmaster 2343bwx (2048x1152 - odd size, I know).
Under Lion, I could run a Windows 7 VM in OS-native full screen mode (with Aero) across all three monitors with absolutely no problems at all.
Since upgrading to Mountain Lion, I've had the exact problem that the aforementioned KB describes.
If I switch to single-monitor full screen mode, it mostly works fine. There are occasional minor artifacts on the non-used monitors, but nothing serious.
Coherence is a crap shoot. It mostly works OK, but then you get some weird flickering and artifacts within the VM windows.
The best solution so far is to disable 3D acceleration. It then runs full screen across all three monitors just fine... mostly. I have a client that uses HipChat, and the Windows client is an Adobe Air app. It struggles with proper refreshing of the screen when another window has been covering it. I'm working on a WPF app inside of Visual Studio 2010 and it all seems to display correctly. (Edit: Chrome on Win7 has the same non-refresh problem sometimes.)
I'm going to try a reinstall of PD7 to see if that helps any.
-Rob
P.S. I sort of expect this sort of thing when a new version of OS X hits the street - there's a lot of very deep integration magic going on for both OSes to get everything working so well. If you think about it, it's pretty amazing that it works as well as it does. I wouldn't be surprised if it wasn't related to some last-minute ATI driver tweaking by Apple for 10.8.