Interested in turning your iDevice into a musical instrument? Well, look here, a framework just for that:
MoMu is a light-weight software toolkit for creating musical instruments and experiences on mobile device, and currently supports the iPhone platform (iPhone, iPad, iPod Touches). MoMu provides API’s for real-time full-duplex audio, accelerometer, location, multi-touch, networking (via OpenSoundControl), graphics, and utilities…
… and on the lighter side, if you haven’t heard of the Stanford Mobile Phone Orchestra, you’ve definitely got to check that out.