This is kind of frustrating. I wish I knew more about the Leonardo HID architecture. Someone already listed the spec above on USB MIDI and it's actually pretty straightforward. The problem is that HID functions don't seem to be easy to get at without some serious hacking. I don't know where I'd begin on this. It seems pretty easy to send HID data (HID_SendReport) but not so easy to register a device... Is anyone on the development team working on this? Seeing as there's already Keyboard and Mouse implemented, it probably wouldn't be a huge project for someone who already understands how to set up an HID driver/register an HID device...? Anyone want to post a little information with where one might try to start on this?

I build a device that used the circuit outlined here: http://arduino.cc/en/Tutorial/Midi

It works great, *but* it doesn't play well using MIDI Thru on another device. Adding 220 Ohm resistor between the TX on the Arduino and the signal pin on the MIDI port fixed this right up. Might be a good idea to add this to the circuit on the website in case other folks have the same problem.