Tmack writes "While hard-hacks with the Wiimote are somewhat old news, this particular implementation is quite interesting. Using the infrared camera on the Wiimote, pens with LEDs instead of ink, and an LCD projector, Johnny Chung Lee of Carnegie Mellon University has created software to use them as a (relatively) cheap multi-touch display. Any surface onto which you can project becomes an interactive multi-touch display, as demonstrated in the video at the link. He has the software available for download, along with some other neat projects.
Lee has also documented another impressive Wiimote hack