I wondered this myself. While technically you could on a generic computer (a wire is a wire, digital and/or analog), there certainly don't exist any APIs to do this on the iPhone. It would make far more sense to hook up to the dock (for which I believe there are APIs for).

The second giveaway is that you definitely cannot get enough power out of the headphone jack to run any serious electronics, not to mention a transmitter that can apparently transmite hundreds of feet in a crowded place like Times Square.