I can't believe I didn't post this already. Based on Remy's comment on the Wiimote interface, here's a similar thing done with a standard webcam by a Microsoft researcher.
And while I'm at it, might as well post this Minority Report type interface using the Wiimote as well. I have to say that the Wii has inspired some crazy-awesome interface hacks that get me excited about what will soon be available. Hey, the hardware and drivers are there, the manufacturing is in place (and cheap!), so all we need is someone to tie it all together. And give me 20%.