Cool project! The eyewriter from FAT labs -- it's the open source eyetracker you're looking for!
http://eyewriter.orgThe indomitable Ben Heck has also made a sip/puff controller: http://benheck.com/03-01-2009/air-pressure-kick-pedal-for-gh... Not sure about ipad peripheral support.
Or, if you want to go another route you could get a 3-axis accelerometer (and/or gyro) to detect head motion/position. It'll output some voltage that you'd feed into the ADC of your microcontroller of choice. Send it all to a python/c#/whatever-you-like script over USB, and process it computer-side. This interface would cost, like, $40!
