Thanks for the heads up on Fritzing - I was wondering how people were making their diagrams. 
Here's what's going on:
https://dl.dropbox.com/u/324863/ArduinoMidi01.jpg
And the sketch I'm messing around with. Here's the un-altered original:
/* Midi Glock - Mike Cook April 2008
*Â based on code by kuki
* -----------------
* listen for MIDI serial data, and fire solenoids for individual notes
#####################################################################################################################################################
HARDWARE NOTE:
The MIDI Socket is connected to arduino RX through an opto-isolator to invert the MIDI signal and seperate the circuits of individual instruments.
Connect the 8 solenoids to pin2 to pin9 on your arduino and pin 13 to the drive enabling monostable.
####################################################################################################################################################
*/
//variables setup
 byte incomingByte;
 byte note;
 byte velocity;
 int noteDown = LOW;
 int state=0; // state machine variable 0 = command waiting : 1 = note waitin : 2 = velocity waiting
 int baseNote = 60; // lowest note
 // use different values of baseNote to select the MIDI octiave
 // 24 for octiave 1 -- 36 for octiave 2 -- 48 for octiave 3 -- 60 for octiave 4 -- 72 for octiave 5
 // 84 for octiave 6 -- 96 for octiave 7
Â
// play only notes in the key of C (that is no sharps or flats) define pin numbers:-
 byte playArray[] =  { 2, 0, 3, 0, 4, 5, 0, 6, 0, 7, 0, 8, 9 };
// corrisponding to note 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48 - for base note = 36 or C2
int strobe = 13;Â // select the pin for the monostable
int channel = 1; // MIDI channel to respond to (in this case channel 2) chnage this to change the channel number
        // MIDI channel = the value in 'channel' + 1
//setup: declaring iputs and outputs and begin serial
void setup() {
 pinMode(strobe,OUTPUT); // declare the strobe pin as output
 pinMode(2,OUTPUT);    // declare the solenoid's pins as outputs
 pinMode(3,OUTPUT);
 pinMode(4,OUTPUT);
 pinMode(5,OUTPUT);
 pinMode(6,OUTPUT);
 pinMode(7,OUTPUT);
 pinMode(8,OUTPUT);
 pinMode(9,OUTPUT);
 state = 0; // initilise state machine variable
 //start serial with MIDI baudrate 31250 or 38400 for debugging
 Serial.begin(31250);   Â
 digitalWrite(strobe,LOW);Â
}
//loop: wait for serial data, and interpret the message
void loop () {
 if (Serial.available() > 0) {
  // read the incoming byte:
  incomingByte = Serial.read();
  digitalWrite(strobe,LOW);  // clear any previous strobe
 switch (state){
   case 0:
  // look for as status-byte, our channel, note on
  if (incomingByte== (144 | channel)){
    noteDown = HIGH;
    state=1;
    }
  // look for as status-byte, our channel, note off
  if (incomingByte== (128 | channel)){
    noteDown = LOW;
    state=1;
    }
   Â
   case 1:
   // get the note to play or stop
   if(incomingByte < 128) {
     note=incomingByte;
     state=2;
   }
   else{
   state = 0; // reset state machine as this should be a note number
   }
   break;
  Â
   case 2:
   // get the velocity
   if(incomingByte < 128) {
    playNote(note, incomingByte, noteDown); // fire off the solenoid
   }
    state = 0; // reset state machine to start     Â
  }
 }
}
void playNote(byte note, byte velocity, int down){
 // if velocity = 0 on a 'Note ON' command, treat it as a note off
 if ((down == HIGH) && (velocity == 0)){
   down = LOW;
 }
//since we can't play all notes we only action notes between 36 & 49
if(note>=baseNote && note<(baseNote + 13)){
 byte myPin=playArray[note-baseNote]; // to get a pin number between 2 and 9
 if(myPin != 0) {
  digitalWrite(myPin, down); // play it if it is one of our notes
  if(down == HIGH) digitalWrite(strobe, HIGH); // strobe high to monostable for note on
 }
}
}
It's a bit frustrating, because it seems as though there are a few people out there, who have been able to use that same sketch without changing anything.