The headset jack of the iPhone and 2G iPod touch provides the proper bias voltage for electret mic in compatible headsets, so you ought to be able to design your own mic that will work. There at least a couple of commercially available microphones that plug right into the headset jack. The bias voltage is around 2.7 VDC and another
forum post indicates that a 3.3 k ohm resistor drops that voltage to around 1.9 V.
Good luck!
Ben
