well, i've been too scared of frying my arduino to have a go doing something like that schematic. BUT! i have some digital capacitance sensors on order, so once they arrive i'll make a prototype and keep you posted. if they EVER arrive goddammit!
i'm not getting my hopes up, though. the qprox folks seem to think that capacitance-in-air applications, like a theremin, really aren't what the QT range are made for. they only ever seem to talk about touch control buttons and fluid levels, yadayada. but we shall see...