Kam points me to this virtual keyboard, which is a miniature projector that you hook up to your PDA; it projects the image of a keyboard onto any surface, and you "type" on it, and the system detects where your fingers are and registers keystrokes.
Of course, given the time of year, this could be a hoax. Either way, cute toy. I suspect it would have the same problems as most unusual keyboards, particularly the lack of tactile feedback as you press keys. But there are circumstances where it could be awfully useful anyway.