This week, I experienced exactly this problem:
http://discussions.apple.com/thread.jspa?threadID=1065874&tstart=0
At first I was sure it was a hardware problem, because it affected the screen both in OS X and when I booted to XP with Boot Camp. I knew the video card was okay, because my external monitor looked fine, but I thought it could be a problem with the laptop LCD, or with the ribbon cable.
But I deleted ~/Library/Preferences/com.apple.universalaccess.plist, just like the last poster said in that thread. Then I did a complete shutdown of the laptop (not a warm boot) and when it came back, it was fine.
Weird.
If you delete com.apple.universalaccess.plist, you will lose any custom keyboard shortcuts you may have defined, but then you can re-create them. Small price to pay.
