I have been working on this myself for the last few weeks (I know nothing about programming so I am sure my code is not optimal) but you need the breakout board and USB TTL interface. I am able to control my iPhone's functions i.e. ffwrd/rew/skip/play/pause and selecting a playlist's and songs within them. I can get all song info for currently playing song. I can get the iPhone's name as well. My only issue is that I canít find a resistor that activates the serial port on the iPhone without giving the airplane more error. I can activate audio out without the message but thatís useless. Basically what you do is send a hex message to put the device into iPod mode then everything else works from there. You need to end each command string with a checksum that basically makes the message = 0x100. For example to get into iPod mode you would send FF 55 03 00 01 04 F8 which is Header (1st 2 bytes 0xff 0x55), Length (1 byte 0x03) that includes Size of Mode + Command + Parameter, mode (1 byte here 0x00), the command (2 bytes 0x01 0x04) ad the checksum (always 1 byte 0xf8) which is the length + mode + command + parameter so it = 0x100. I can attach my crappy code if anyone wants to try taking a look at it and if I can figure out how. Just if you make something you decide to sell please remember me. :-P
By the way this is a great site to get info from. http://www.nuxx.net/wiki/Apple_Accessory_Protocol
Here is what I have so far, like I said before I know nothing about programing so please fogive me if this is not in standard layout. It is a c# 2010 express project that is a stand alone forms application as of now but the idea is to use what I learn doing it to make a plugin for the major front ends.
I'll look at it later. I can't stand c# and I don't have visual studio installed. Here is mine, for what it's worth. I haven't really touched it in over a year, but it worked. It's tested on linux and works, on windows it compiles but hasn't been tested even once. It does everything over stdin and stdout, you are meant to tie it in to your own programs yourself. I have a jukebox application I wrote that lets me control MPD with a crystalfontz 4 line display with keypad, and I modified it to call this program and give it ipod abilities. Best of luck....