Some time ago [ant.b] from the Arduino Forum manage to reflash the Atmega8u with other LUFA Firmwares. [Dimitri Diakopoulos] has recently developed a similar approach for his HIDUINO project. Very good explanation & references. Diakopoulos succeded to make Arduino show up as a MIDI Device:
LUFA powers the HIDUINO project in that it handles most of the low-level USB-HID implementation while exposing an API for developing other HID-compliant devices like MIDI.
The USB-HID specification has a specific type for MIDI input and MIDI output, which nearly all commercial musical controllers on the market use for class-compliant (driverless!) MIDI I/O.