I wrote a native MIDI program for 8u2 chip on Uno.
Moco for LUFAhttp://bit.ly/gMKhAy
You can replace original USB-to-serial firmware with Moco. This driver will bridge between USB-MIDI and serial-MIDI.
(1) The host will recognize Arduino Uno as USB-MIDI device without any driver on MacOSX and Windows XP.
(2) You need to write original USB-to-serial firmware back to 8u2 for the reprogramming of main CPU.(atmega328)
As the driver is USB-MIDI class compliant, iPad can recognize Uno as MIDI device.
I hope the program is useful for arduino community.