The only thing occurring here and there is this weird mirror effect on the dock. (As seen below.) Hopefully 10.5.2 is going to fix this. When it happens, all I have to do is restart it and fixes itself. But I haven't been getting any freezes so far... What's even odder is that you can't take a snapshot of the screen when it happens. Had to use a camera.
Very interesting... It looks like the bottom 20 (or so) lines of pixels on the screen is displaying the same line, over and over again, rather than what is supposed to be there.
Unlike the problems you were having with your previous Mac, this could very well be a driver issue. If this isn't fixed in 10.5.2, you've got to hassle around with Apple to make sure they are aware of the issue so that it will get fixed soon.
I'm curious if it's possible the monitor is playing a roll. Next time it happens, see if just resetting the monitor but not the computer makes it go away.
Try
1. Turn monitor off and on.
2. If that doesn't have an effect, unplug the monitor cable from the MP and plug it back in.
3. If that doesn't have an effect, unplug the monitor cable from the MP, then unplug the monitor from the wall, wait ~10 sec, plug it back in to the wall and then plug the monitor cable back in to the MP.
#2 and #3 will probably cause the OS X graphics driver to reset to some extent, so I'm not sure what they prove, monitor vs. driver-wise.
but if none of these affect the problem, then it's got to be the graphics driver, OS X, or (possibly) a GPU hardware problem.
Oh yeah, does jiggling the cable connecting the monitor to the MP seem to have any affect on the issue? Seems really unlikely given the symptoms you're having, but the cable is always something to consider...
Anyway, I'm on the verge of buying a MP just like yours, so I have a direct, personal interest in you resolving this! Good luck...
Oh, one more thing: If you drag something over the affected area of the screen, is the corruption "live?" That is, does it change as you drag something around it? Or does it look like it gets overwritten with the correct pixels? Or is it "dead" -- nothing you do has any affect on those pixels?