Why does the gyroscope in an iPad stop working on an airplane?
OK I THINK I FIGURED IT OUT!
It was my fault almost entirely, but I have an answer that may be useful to others.
Here is the deal:

1: I set the gyros to update at 60 hertz
2: On the ground, the app RAN at 60 hertz, so grabbed one gyro update per frame
3: In flight, my app did MORE WORK (!!!) to display what the airplane was doing, and thus fell to less than 60 hertz update rate of the app!
4: At this point, the update messages from the gyro queued up and the event-reader got behind... WAY behind, so I was looking at real gyro data from EARLIER IN THE FLIGHT!

So, the REAL lesson here is that setting the update rate of the gyros to a value greater than the frame-rate of the app results in event queing that has you running behind.

If anyone knows how to soak up ALL the gyro events EACH frame of the app, rather than just reading one, then that would APPEAR to solve this issue... whew! Interesting!