Years ago, along with my colleagues, I began to follow the work of the NUI Group on the implementation of multi-touch screens. Later I participated in the development of some prototypes.
The first prototype was based on the technique of FTIR (Frustrated Total Internal Reflection) and the following on the DI (Rear Diffused Illumination). It was chosen the FireFly ® MV CMOS Camera, at the time only available in IEEE 1394 FireWire.
This PDF from NUI Site explains in detail all the technology tested. Multi-Touch Technologies
TouchLib was the first working engine for the recognition of the Blobs (reflected image from the fingertip). Since the OS used was Windows, after I modified the code of TouchLib to specialize it to use Directshow code. Directshow allowed to enable operation mode of the camera more efficient. How to increase the frame rate to 60fps with a not-significant reduction of the resolution.
A trial ended NUI has released a new engine Blob Tracking, also based on NVIDIA’s CUDA, called CCV (Community Core Vision).
In the last prototype was a Laser Light Plane (LLP) with good results. I had a pleasure to burn the eyes of my colleagues. 
The biggest problem of this technology, that does not make it resalable for commercial application outside of exhibitions, interactive museums and pubs, is incompatibilities with any direct light including the normal sunlight. 
For these reasons, I started an experiment based on the construction of a small array of IR sensors, but not based on the simple photovoltaic (PV) detection, but on a demodulation process of the IR source.
The same system adopted by the IR receiver on the TV. You may have noticed that your television will continue to receive commands from the remote control correctly even if you are using it outside in the sunlight. 
I made an initial prototype for testing the capability of scanning for a middle-class micro at 20Mhz clock. In this prototype, the detection is done simply in PV mode.
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The purpose of this prototype is to verify the functionality of the technical choices, seen in low number of IR phototransistor and high distance between them.