Step 1: What you will need

A multitouch interface is not terribly difficult to construct but there are many supplies and you should know what your getting into. It is easy to get wrapped up in the idea of what kind of software you could make for multitouch, forgetting that you will need an interface if you want to make that software.

So here is a list of the supplies you will need.
Computer with decent processing speed
Digital projector
Webcam
Surge protector
InfraRed LEDs (depends on the size of your table but think around 50 of them for good measure) I am using some I got from Mouser
Plexiglass (cut to the size of your touching surface)
Silicon Rubber, I am using Near Clear
A material to make the table, I am using a medium grade plywood
Mirror to reflect projection upward
Something to hold the LEDs, I am using some C channel aluminum
A projection screen, I am using vellum paper

Minor supplies:
Roll of film, any 35mm film will work.
Sandpaper (and hopefully a sander)
Tools to make the table
Wiring and a power supply for LEDs
Speakers, this isn't necessary but I am including them

Supplies you might need:
Reflective Tape

So now that we have our supplies, lets get to work.