"The team behind Project HiJack envisions users building low-cost sensing and data acquisition systems for student and laboratory use. So far, it has built an EKG interface, soil moisture sensor, an integrated prototype with temperature/humidity sensors, PIR motion sensor, and potentiometer, and a version with a breadboard for prototyping new sensor applications.
Schematics for the HiJack board, as well as source code to enable communication via the audio port, are available on Google Code so that anyone with some soldering skills and the wherewithal can build a HiJack for his or her own use. Currently, software exists to work on iOS, but the hardware design should work with nearly any mobile device that has a combination headphone/microphone jack. The team plans to build APIs to enable the HiJack to work on Android and Windows Phone 7 in the future." More info and video at ars technica