Like Ars Technica mentions, creating iPhone accessories using the 30-pin connector is costly because companies have to pay Apple a fortune in licensing fees (it's their proprietary tech). That's not the case with the headphone jack. Enter Project Hijack.
A team of engineering students and faculty at the University of Michigan have developed an open protocol for using the headphone jack as an accessory port. Low power devices in the past, such as the Square transaction adapter currently use the headphone jack, and Project Hijack aims to utilize the port in a similar fashion. Ars says the technology can convert a 22kHz audio signal into 7.4mW of power running at 47% efficiency, which in turn powers a TI MSP430 microcontroller. What you do with this bridge after that is up to you.
This is mostly intended for those looking to build low-power, sensor-based accessories on a budget. Because this can be mass produced for as low as $2.34, and side-skirts the issue of licensing fees, plenty of dollars can be saved. So far, they've used Project Hijack to create EKG monitors, potentiometers, motion sensors and moisture/humidity sensors.
All the schematics and necessary info are currently available on Google Code for you to build yourself. They also plan to produce 20 of these devices to give away to those who submit good ideas. [UMich and Google Code via Ars Technica]