The multitouch trackpad on a macbook pro [1] feels very consistent. This is key for an instrument. It can be as responsive as you want, but if it's not consistent you can't play it automatically. I found an existing trackpad synth, but it wouldn't let me change the note selection interface. So I wrote one. It's still very much alpha, but the code is on github. It's four octaves, diatonic, but that is adjustible in the code.

It works by slightly extending some multitouch sample code to output SKINI which I'm using the Stanford Synthesis Toolkit to turn into audio. My next target is polyphony.

[1] which I now have for my new job.