You can replace the atmega16u2 firmware with a USB midi driver and you get native USB midi in and out of your arduino. Take a look at my blog here Arduino UNO Midi firmware and use the latest firmware from here. Also check out the other projects doing the same thing on the playground: Arduino Playground - InterfacingWithHardware.
The drawback is that you can't yet use serial and Midi at the same time so you have to burn back the serial driver to load sketches or use the ISP.
If you're using Linux or OSX see this thread about patching dfu-programmer to support the atmega16u2 http://arduino.cc/forum/index.php/topic,92148.0.html. If you're on windows then the latest version of flip from Atmel should already support it.