The height I was at it was able to pick up 3G and at points GPRS and EDGE however if you were higher you would need in flight wifi, although I tried the GPS out of signal and it seemed to come up with a dot but no map
I've never had any GPS unit work for me in a plane and not for a lack of trying. Or most other vehicles such as a train. I'm guessing the reason is the metal body, wiring, insulation, frame, bulkheads, etc. Even if you sit by a window, the signals would be so weak or so few that it wouldn't work. Probably the best you can do is to book a flight on an airline that shows you where you are on a map on the little TV screen in front of each seat. I know United and BA have that on international flights.
Ok, this isn't an iPhone, but it's a GPS working on an airplane! This is a USB GPS unit stuck to the window. Worked great. If you can't read it, that's 406 mph at 41,000 feet.
Check out the two photos.
NO GPS AND CELLPHONE DEVICE ON THE PLANE!
I am calling FAA and Home Land Security on your ass...