The most important part of this setup is the voltage divider
. The iPod Rx only expects 3.3v, so anything more than that (like the Arduino's 5v TX) could hurt your iPod. The voltage divider is a safe way to ensure that when the Arduino sends a 5v HIGH, your iPod only gets hit with a 3.3v HIGH.
The divider is accomplished in two steps. First tie the Arduino TX (pin1) to a 1k ohm resistor. Second, on the other end of the resistor attach iPod pin 13 (RX) AND two 1k ohm resistors (in series).
The 500k Ohm resistor attached to pin 21 tells the ipod that you will be using a serial communication (see reference
Both iPod pins 1 and 11 should be grounded to the arduino. (To the nit pickers, I know the references state different things about pin 11, but grounding it seems to have no effect on this project.)
The rest of the hardware should be pretty straight forward, if you'd like more information on push buttons there is a tutorial on the arduino website