MaximumPC has step-by-step instructions to build your own Surface Computer. Check out the video, looks like they’ve done an excellent job at creating a responsive systems:
Maximum PC’s Multitouch Surface Computer from Maximum PC on Vimeo.
Their approach uses an FTIR (Frustrated Total Internal Reflection) method, which is quite commonly used in the literature. Another technique, shown here by Paul Foster uses an IR rear-diffused method. I’m partial to FTIR because of its potential responsiveness, but I’m not sure if that’s born out in reality. Both approaches are great homebrew projects.
Both groups use software from the NUIGroup–a must have resource for any multi-touch experimenter.