well, for anyone interested... i've had some initial success with this project - in that i've somehow managed to get a QT300 capacitance-to-digital sensor talking to arduino in a vaguely meaningful manner. yay! you can see some of the progress and read about what needs to be done next on this thread (titled "supply current..."):
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1169728880
hopefully you might also be able to emulate what i've done, if you're interested, but otherwise i'll do my best to write up the proceedure as a tutorial soonish...
i'll start a new thread once i'm back from a (potentially very fun and messy) beach roadtrip and have had time to get round to investigating ways to make the setup behave more like a theremin should behave. at the moment it's actually quite bad, but i've got a few ideas on how to improve it. if you do too, i'd love to hear about it!
love, jon.