I've actually been building it since December, but I've only now gotten round to setting up a blog to record the process.
A while back I was messing around with using a webcam in Processing to track movement and ended up happening across the fiducial system employed by Reactivision for their rather spiffy Reactable.
I'd originally planned to build a fiducial driven coffee table to use in reception at work for displaying staff profiles, case studies and the like. However, it quickly became apparent that the fiducial markers would obscure quite a lot of the screen and I'd need to make a massive table to accommodate both the markers and the information I wanted to display.
After a spot of googling research I happened across the rather splendid work being carried out by NUI Group and the many talented denizens of their forums. Here were a group of individuals who, while everyone was marvelling at seeing Jeff Han's TED Talks demos, were actually building their own multitouch systems.
Inspired by what appeared to be a return to the heady days of building your own computer (I don't mean going to dabs and buying components. I'm thinking more of the type of thing Woz was up to in the days before Apple was incorporated) I felt compelled to join in the fun and build my own multitouch system.
The majority of multitouch systems use a projector for the display, but projectors are expensive and I wanted something a tad more reasonable for a my first attempt. A couple of the NUI Group forum members were building multitouch systems with LCD monitors instead of projectors and I was fortunate enough to acquire a free one from work so I was off and running.