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.

Check out guide for ISP flashing, on the project page hosted on googleCode.

via [HackADay], source [HIDUINO]