Betcha didn't know your iPhone has a secret way of powering and controlling external devices, did ya?
It's the audio jack. And a group of hardware hackers at the University of Michigan are taking advantage of it. Rather than decypher the mystery of Apple's proprietary 30-pin port — and perhaps taking a cue from the folks at Square and Seeing Red — UM's "Project HiJack" harvests both power and data bandwidth from the 3.5mm headset jack to run cubic-inch peripherals. Their web page includes step-by-step construction pictures and some example application videos (for monitoring EKG's and measuring soil moisture).