From the MAKE Flickr photo pool
Flickr user [fluxa] put an Arduino board to work outputting control voltage to a DIY SoundLab synthesizer.
While the PC software has yet to be released, the Arduino code is pretty straightforward – using a pulse width modulated output through a simple resistor/capacitor filter. – CVAirSequencer Project on Flickr
Hmm … If that R/C filter slows down response time too much, one could always use a simple SPI or R2R DAC, and forego the PWM altogether.