I've had this happen twice now with a brand new 13" MacBook Pro, 2010 model.
Both times it has happened I've had an external display connected. The external display turns grey, the internal display turned purple once, dark green another time.
The second time it happened I tried using SSH to connect to the computer. Worked fine, the computer was totally responsive. I was able to reboot the computer from the command line to get it back to normal. So it seems to be a display issue alone.
I'm currently using 10.6.3—I'm about to install 10.6.4 now. I don't know if that will make any difference, but I do get the feeling it's a software or firmware issue rather than a hardware defect. If 10.6.4 doesn't help I guess I'll be taking it in to Apple.
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Here's another person with what sounds like the same issue:
http://discussions.apple.com/thread.jspa?threadID=2441287
Worth noting: at one point I tried overtaxing my computer to force the issue to happen. Opened probably 40-50 apps, processed a 4GB file in Photoshop, etc. Nothing happened. For me at least, I'm confident it's not an issue of overheating or anything. I think both times it happened I was just doing light work like using Safari, browsing files in Finder, etc.
Message was edited by: robotspacer