Georg Kaindl the developer of the Open Source multi-touch tracking software for OS X Touché has published a blog post about how a laser light multi-touch setup can be build on your own.
He has also published a video in which he presents how the laser light multi-touch table works:
DIY laser multi-touch table – Raw camera feed from Georg Kaindl on Vimeo.
Update: Some more links:
Nuigroup LLP Wiki page
Alex P’s initial LLP setup at the nuigroup