I am interested in the matter as well. My friend's iPhone didn't work on my Mini^3 when I wanted to show off how much better sound could be, but then it didn't work (to my shock but not to my surprise). Anyway, this is the best I could find.
Mike Kruckenberg: iPhone Dock Extender Cable (with USB data sync)
Read the comment. It doesn't make all that much sense to me, but since you have an iPhone in front of you, you can test it out yourself and interpret what dude is saying.