I saw a project once where they used acrylic sheets with touch-- they had a camera on the back side with the IR filter removed. Fingerprints on the outside showed up as hot spots on the inside. Just something else to consider. Wii-motes have IR cameras in them and are bluetooth enabled... I bet you could point one at each side of the building, from the inside. (from there, I don't know what you'd do with it though!)
There's also a project with Wii-motes where a guy wrote software to convert IR points from the Wii-mote camera into gestures on a screen. The initial calibration tells the software where your "screen" is in relation to what the Wii-mote can see.
Jeremy