Sometimes when I move between spaces, the little animation freezes (doesn't fade away), and it blocks out all keyboard access. I can still move between spaces (though this usually doesn't unfreeze it), and use the mouse.
I used to fix this by quitting the Dock with Activity Monitor (which was annoying because I had to do the whole thing only with the mouse), but today I discovered that if I press the Dashboard key on my keyboard (it's F12 by default), it unfreezes it (and it doesn't even open dashboard).
[crarko adds: I haven't tested this one. Any ideas about why this should be the case?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2011061520233531