I can not express how relieved I was to see this thread. I started my own last night (
http://discussions.apple.com/thread.jspa?threadID=2420479&tstart=0) about the exact same issue! Until now, people were telling me it could be my GPU going bad but this thread has proven to me it has got to be software. And switching to a guest account won't fix it because it's a driver issue with 10.6.3 It does this when I watch videos on youtube, etc. Surprisingly hard to make it happen when watching Quicktime videos like .avi or .mov files.
I installed SL back in September and right away noticed the glitching shadows. But it went away until a few weeks ago and bam- same thing as everyone else here. I seem to have done all the stuff people have suggested: reset the PRAM, sys management, verified the disk, Apple Hardware Test, uninstalled Flash Player, and use a 2D dock.
I monitor my CPU temp with iStat and it's around 100 degrees F.
Made a Picasa album with screencaps made my bg white to really show off the glitching:
http://picasaweb.google.com/nickux/MacScreenArtifacts?feat=directlink
Here's to hoping we get a fix in 10.6.4!!