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There are existing projects that allow the Uno and other devices using the ATmega8u2 to operate as a USB MIDI HID, such as MocoLUFA. Unfortunately, as far as I am aware, these require uploading new firmware that isn't compatible with programming via the Arduino UI.
Are there any similar projects for the ATmega32u4 (Leonardo, Micro)?
Is it possible to use an Arduino as a USB MIDI HID, while still being able to program it as normal?
The teensy series from pjrc.com allows USB MIDI natively (full USB speed!). It works well and does not require any firmware tomfoolery. It can also do HID keyboard/mouse/joystick natively as well.
There's the 2.0, which is 32u4, and the 2.0++ which has more pins and memory. If you want to step off the Atmel reservation the teensy 3.0 and 3.1 have more speed and memory; in a couple of weeks they are coming out with a Teensy LC which is a 3 with less bells and whistles for cheaper.
