I'm cross-posting this on the Sparkfun forums, but I wanted to have an Arduino-specific explanation here. I created some code that allows the Arduino to control the NS73M FM transmitter (available on a breakout board from Sparkfun). The biggest problem I faced in making this work was that the NS73M communicates over SPI, but it needs a 4 bit register address and then 8 bits of data for that register. The Arduino SPI implementation (as well as the ShiftOut command) seem to be hard coded to send 8 bit data which wouldn't work. I had to do a little bit of bit-math to try to fake SPI. This is based very much on the example code provided by Sparkfun.
This is my first real Arduino project, so if anyone has any suggestions or improvements, I'd love to hear about them. I used the schematic above for hooking up the Arduino to the NS73M except that I connected the clock line to pin 12 instead of pin 13 to avoid any issues with the LED on pin 13.
This is only the first part of my project which I'll probably post in the "Exhibition" forum once it's complete. I'm calling it ARRRduino! 
/*
12-1-2007
Cai Maver, caimaver(at)yahoo.com
ARRRduino+NS73M v0.3
This code allows an Arduino Diecimila to control an NS73M FM Transmitter Module (Arrr matey!)
This sets the NS73M (available from Sparkfun, SKU#: WRL-08482) to transmit
at 2mW with 75us pre-emphasis (the standard for North America) on 97.3 MHz.
Use this formula to determine the register values for a new transmitting frequency (f):
(f + 0.304)/0.008192 <-- use only the whole number and convert the result to
16-bit binary where the lower byte goes in register 3
and the upper byte goes in register 4
ex.: for 97.3 MHz, (97.3 + 0.304)/0.008192 = 11914.55...
11914 = B0010111010001010 so Reg 3=B10001010 and Reg 4=B00101110
A future version of this code will allow for on-the-fly frequency changes
Thanks to Nathan Seidle at Sparkfun and Jim "ZAPNSPARK" G. for sharing their NS73M code!
*/
int CK = 12; //clock pin
int DA = 11; //data pin
int LA = 10; //latch pin
void setup(){
Serial.begin(9600); //begin Serial connection for debugging
pinMode(CK, OUTPUT);
pinMode(DA, OUTPUT);
pinMode(LA, OUTPUT);
digitalWrite(LA, LOW); //unlatch transmitter
delay(100); //Wait for VDD to settle
spi_send(0x0E, B00000101); //Software reset
spi_send(0x01, B10110100); //Register 1: forced subcarrier, pilot tone on
spi_send(0x02, B00000011); //Register 2: Unlock detect off, 2mW Tx Power
spi_send(0x03, B10001010); //Register 3: Set broadcast freq to 97.3, lower byte
spi_send(0x04, B00101110); //Register 4: Set broadcast freq to 97.3, upper byte
spi_send(0x08, B00011010); //Register 8: set Osc on band 2
spi_send(0x00, B10100001); //Register 0: 200mV audio input, 75us pre-emphasis on, crystal off, power on
spi_send(0x0E, B00000101); //Software reset
spi_send(0x06, B00011110); //Register 6: charge pumps at 320uA and 80 uA
Serial.print("Transmitting"); //for debugging
}
void loop(){
}
void spi_send(byte reg, byte data) //routine to send Register Address and Data as LSB-first SPI
{
int x;
int n;
digitalWrite(LA, LOW);
for(x = 0 ; x < 4 ; x++) //send four-bit register address
{
digitalWrite(CK, LOW); //Toggle the SPI clock
n = (reg >> x) & 1; //n is the xth bit of the register byte
if (n == 1){
digitalWrite(DA, HIGH); //Put high bit on SPI data bus
}
else {
digitalWrite(DA, LOW); //Put low bit on SPI data bus
}
Serial.print(n); //send bit to serial connection for debugging
digitalWrite(CK, HIGH); //Toggle the SPI clock
}
for(x = 0 ; x < 8 ; x++) //send eight-bit register data
{
digitalWrite(CK, LOW); //Toggle the SPI clock
n = (data >> x) & 1;
if (n == 1){
digitalWrite(DA, HIGH); //Put high bit on SPI data bus
}
else {
digitalWrite(DA, LOW); //Put low bit on SPI data bus
}
Serial.print(n); //send bit to serial connection for debugging
digitalWrite(CK, HIGH); //Toggle the SPI clock
}
delayMicroseconds(1); //might not be needed, supposedly unstable command anyway
digitalWrite(LA, HIGH); //Latch this transfer
delayMicroseconds(4);
digitalWrite(LA, LOW);
digitalWrite(CK, LOW); //This is to keep CK pin at 0V at end of data transfer
Serial.print("\n"); // send new-line to serial for debugging
}
