Just thought I would put together a list of all the MIDI stuff I have stumbled on in my quest to build a MIDI controller:
Arduino:
"Arduino basics while building a MIDI controller"
http://roguescience.org/wordpress/?page_id=33
Arduino MIDI Library
http://timothytwillman.com/itp_blog/?page_id=240
USB-MIDI on the Arduino
http://tomscarff.110mb.com/USB_MIDI/USB_MIDI.htm
If you want to go Wireless
http://www.ladyada.net/make/xbee/midibee.html
If you need more inputs than the standard Arduino, you can of course use the Arduino Mega or the Teensy with Teensyduino (Basically an Arduino Clone)
http://www.pjrc.com/teensy/
http://www.pjrc.com/teensy/teensyduino.html
The nice thing about Teensy is you dont have to use Teensyduino, you can code in straight C along with LUFA, and make it true USB-MIDI.
LUFA Examples:
http://www.noisybox.net/electronics/dickhole_keypiss/
Anything else?