This is not the thermin idea exactly but works a lot like one. A theramin, as I understand it, uses two radio frequency oscillators, and the audio signal is the beat frequency between them, that one controls by detuning one of the oscillators.
The reference below is a capacitive sensor that operates in low radio frequency
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1171076259
Paul B