Here's some useful links I book marked:
I was planning on building an interface... the theory behind it is pretty simple... But I'm going headunit-less now and it seemed pretty pointless to carry on.
Basically, you need to reverse the protocol so you use your PC to pretend to be an Ipod.
Then the commands the unit sends out will be received by the PC and you can do as you like.
The difficult part is interfacing the PC so that song titles, etc. are sent to the head unit.
The protocol itself is pretty straight forward if you've had some programming experience.