I just successfully tested code and breadboard prototype for a MIDI keyboard encoder. It utilizes 8 74LS151Ns so the switches it's scanning can be parallel-wired (opposed to a matrix). It scans 64 switches, although my design intent is for 61-note organ manuals. As of this post, I have only tested by grounding the inputs on each 74151.
I am utilizing the 3.0.1 MIDI library, you can find the link to the most current version at: Arduino Playground - MIDILibrary. The 74151 scanning logic I pulled from the playground at Arduino Playground - MUX151. I also pulled ideas (but no copy/pasting) from chloris's Yet Another Arduino MIDI Drum Kit thread in the Interactive Art subforum http://arduino.cc/forum/index.php/topic,54927.0.html Big thanks to everyone who coded those, I am a total noob at C++ and your sketches helped me make this work.
I do not have a schematic drawn up for the external hardware. It is basically the 8 multiplexers wired to pins 6 through 13, control pins 2-5, and serial TX wired to a DIN jack through a 220 ohm resistor, ala MIDI standards.
#include <Compatibility_v2.5.h>
#include <MIDI.h>
//mux pins
#define mux8 13 // digital inputs to arduino from mux
#define mux7 12
#define mux6 11
#define mux5 10
#define mux4 9
#define mux3 8
#define mux2 7
#define mux1 6
#define strobe 5 //digital outputs to control mux
#define cee 4
#define bee 3
#define aye 2
//array for played notes
volatile int note[64] = {LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,
LOW,LOW,LOW,LOW,LOW,LOW,LOW,LOW,};
//array for scanned switch state
volatile int swst[64];
//arrays to control 8 mux inputs
int c_bin[]={LOW,LOW,LOW,LOW,HIGH,HIGH,HIGH,HIGH};
int b_bin[]={LOW,LOW,HIGH,HIGH,LOW,LOW,HIGH,HIGH};
int a_bin[]={LOW,HIGH,LOW,HIGH,LOW,HIGH,LOW,HIGH};
//extra variables
int select=0;
int a_val=0;
int b_val=0;
int c_val=0;
void setup() {
MIDI.begin();
pinMode(mux1, INPUT);
pinMode(mux2, INPUT);
pinMode(mux3, INPUT);
pinMode(mux4, INPUT);
pinMode(mux5, INPUT);
pinMode(mux6, INPUT);
pinMode(mux7, INPUT);
pinMode(mux8, INPUT);
pinMode(strobe, OUTPUT);
pinMode(cee, OUTPUT);
pinMode(bee, OUTPUT);
pinMode(aye, OUTPUT);
}
void loop() {
scanning ();
notewrite ();
}
void scanning (){
for(select=0;select<=7;select++) {
//select mux input
a_val=a_bin[select];
b_val=b_bin[select];
c_val=c_bin[select];
digitalWrite(aye,a_val);
digitalWrite(bee,b_val);
digitalWrite(cee,c_val);
//strobe LOW to read
digitalWrite(strobe,LOW);
//read mux and store switch state to memory
swst[select] = digitalRead(mux1);
swst[select + 8] = digitalRead(mux2);
swst[select + 16] = digitalRead(mux3);
swst[select + 24] = digitalRead(mux4);
swst[select + 32] = digitalRead(mux5);
swst[select + 40] = digitalRead(mux6);
swst[select + 48] = digitalRead(mux7);
swst[select + 56] = digitalRead(mux8);
//strobe HIGH to avoid jitters
digitalWrite(strobe,HIGH);
}
}
void notewrite () {
for (int i=0;i<=64;i++) {
if (note[i] != swst[i]) { //detects switch that has changed state
if (swst[i] == LOW) {
MIDI.sendNoteOn (i+36,0,1);} //zero velocity aka note off
else if (swst[i] == HIGH) {
MIDI.sendNoteOn (i+36,100,1);} //100 velocity note on
note[i] = swst[i]; //sent note written to array
}
}
}
Please have a look and give me some crap for my noob code 
I plan in the future to implement MIDI THRU, or possibly send MIDI up the USB jack. If anyone knows more about the MIDI library, I would love to hear from you, I don't know how to code the MIDI IN/THRU 