I haven't used the QT touch wheel but have used other of the Quantum sensors. In general the simple ones are easy to use, the grid based sensors in my experience are trickier, and more fuss, but this is probably as one would assume.
I did some experiments with capacitive sensing with the Arduino alone and it turned out to be easier than I would have expected. http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1171076259
I had the idea that it might not be too much of a hassle to implement a scroll wheel with just an Arduino. You would only need one "send" pin and maybe four or five "receive" pins, all linked to the send pin with 1 M resistors. You would then send a "burst" (cycle of square waves) from the send pin, to each receive pin (in order). After reading each pin (see the reference) it would be flipped to an input with pinMode().
Then some logic would be required to tell what the user wanted to do - this might actually be the hard part.
If others are interested I'd be willing to try this out and post the code.