Ipod/Iphone/Ipad cable hack
Pinouts.ru has a great description of the connector.

The iPodLinux wiki has a great page describing the serial protocol.

Here is the connector diagram (with the blank side of the connector facing up):



Pins 12 and 13 are Tx and Rx, respectively. Rx and Tx are TTL level. Pin 1 is GND, and pin 21 is serial enable. To enable the serial port, tie a 6.8 kΩ resistor to GND.