Mac OS X 10.6.3 has a bug when mirroring the primary display to an external display, where if you use an application to play video full screen, the graphics on both displays may become garbled.  This has been noted to affect some MacBooks, MacBook Pros, and iMacs.  When this occurs, the Mac must be rebooted.  Apple is aware of the problem, and is working on a fix.  Until then, workarounds include using extended Desktop mode for external displays (not mirrored), or reverting to Mac OS X 10.6.2.