The technique behind Touché is known as Swept Frequency Capacitive Sensing (SFCS): at a glance, by monitoring the capacitive response of an object over a specific range of frequencies (instead of a single one), it is possible to infer about its interaction with the outside world.
In his blog, Dzl describes his personal approach toward the development of a system capable to emulate Touché’s behavior with Arduino. Currently, the project is still in a early stage, but improvements and further developments are expected soon.
More information can be found here.
UPDATE 2012-06-02: you can now try out how to make it yourself following this instructable.