Someone recently asked if the Arduino could do Multi Touch and it looks like Dangerous Prototypes has the answer.
Open source Multi Touch, known as OpenMT, is an open source hardware project for developing multitouch applications. Unlike camera-based multitouch, OpenMT lets you make a thin, flexible multitouch surface. It is Arduino friendly and has been developed by Sensible UI in Seoul, Korea. Users can experiment with hardware development themselves with projects such as ArduMT, or can use OpenMT software to interface with other multitouch devices.