I've had a few reports of this from Tiger users. I can't duplicate it myself - it seems to be video card dependent, and it seems to be fairly rare (I'll probably get a bunch of "me too's" now!). I assume it has to do with window server compositing of windows, as it happens only when dragging or resizing windows.

ShapeShifter 2.1.1 was an improvement over 2.1 for Carbon apps, and I hope to still make more improvements in a future version for Cocoa apps, even though I don't experience the issue myself.