I've been noticing interesting behavior when parallels6 is running on a MacBookAir (late 2010) where the video on an external monitor becomes distorted. Looks like jaggy distortion of the image. It seems to get much worse when the VM CPU utilization is high. When running Win7 disk defragment tool the jaggy distortion is almost constant. I tried running a CPU intensive benchmark on the OSX side and didn't see the problem. I also paused Parallels VM during the Win7 defragment and the screen distortion stopped; it returned when I resumed. So it seems pretty clear there is some bizzare interaction with Parallels6 and MBA graphics.
