hi,
im looking for information on the same thing!
the shield is not an option because i want to send midi over the Uno's USB port,
from what ive learnt so far, you need to either short some pins or connect the ICSP jumper (the 6 upright pins on the board that are not part of the I/O pins) and then reprogram the atmega which is between the Atmega 328 (which is the main chip on the uno) and the USB port,
you reprogram it in LUFA, to convert serial to midi data, which apparently the LUFA library already has these protocols you just need to implement them, (but i still havent worked out that bit)
and then as long as the Uno is accepted by the generic driver for the USB class as a midi device you should be good to go!
but any more information from anyone on this subject would be greatly appreciated!