Jabawocky,
The existing iPhone (3, 3G & 3GS) uses accelerometer sensing only, and is therefore completely useless to measure anything other than what is up and down when attached to an object in a steady state. As you saw for yourself - completely useless for an aircraft in motion. The big news with the iPhone4 is that it has an ACTUAL 3-axis gyroscope (most probably a laser ring-giro, even though they are not saying yet) built in. A completely different beast altogether, and now completely useful as a device to measure attitude for an object in motion, such as an airplane in flight.
Trust me - this is a game changer.
RJ