Arduino-iPod-Control is a project mainly written in ..., it's free.
Control an iPod or an iPhone with an Arduino's serial port
Readme is a WIP
This code is based entirely off code by crimsondynamo329/Ayelix on ipodtouchfans.com, which was in turn based off code by Rosie Daniel
Eventually I can see this code changing drastically, but right now I claim no credit.