By Matt Mets at Make
http://blog.makezine.com/2010/12/02/make-it-last-build-series-a-metal-b/
Arduino MIDI input circuit
Opto-isolator plus diode
from notesandvolts.com
http://www.notesandvolts.com/2012/01/fun-with-arduino-midi-input-basics.html
Arduino Midi shield
Arduino MIDI
Midi control using 1 resistor.
I substituted 330 ohms for 220 and it worked fine.
http://arduino.cc/en/Tutorial/Midi
here’s the pinout data for sparkfun midi jacks:
http://www.sparkfun.com/datasheets/Prototyping/Connectors/MIDI-RA.pdf