“Pretty much all the devices that are available to consumers such as tablet PC’s, or businesses such as the smart boards and kiosk PC’s are single touch only. The devices just emulate normal mouse interaction with a single cursor. The touchtable, surface pc and perseptive pixels devices all allow you to interact with multiple touches at the same time, either both hands or several people at once. This bit is the most challenging, both from a hardware sensing, and a software processing perspective. I’m not too sure how the touchtable and perseptive pixels stuff works, but the Surface PC works using IR sensing, as decribed here.”

http://geometrybag.wordpress.com/2007/06/15/build-your-own-touchtable/

“Anyway, after a bit of searching I found this site dedicated to building your own touch table using the same principles, there is a diy guide to putting together the hardware, as well as links to an opensource effort to build the software to interact with the hardware”

microsoft surface