Isn't this already supported in most modern MIDI keyboards in the form of Aftertouch? If that's not what you're looking for, you could always get one of these:

    http://www.cerlsoundgroup.org/Continuum/

That's exactly what I was thinking of. Thanks.

(Of course, at >$3000 I'm not actually going to buy one. Just geek out over it.)

Not MIDI, but CV: Ondes Martenot / The French Connection

http://en.wikipedia.org/wiki/Ondes_Martenot

I went to a concert based around that instrument last year in Paris - it's a very weird sound.