Over at Hackszine Jason writes:

This Guitar Hero mod posted by Youtube user Egyokeo blew my mind. A MIDI drum kit and some custom software on a PC send button-press input via the Xbox Input Machine (XIM) hardware to an Xbox running Guitar Hero.

Watch the video and prepared to be amazed as Egyokeo drums to "Through Fire and Flames".  Link.

Original posts:
MIDI Hero: Play Rhythm Games using any MIDI Instrument
Building your own Xbox 360 Input Machine (XIM)