Try attaching your cell phone to a rocket travelling over 1200 mph or above 60,000 feet, you'll find that your GPS unit basically shuts down. That's not to say that your GPS hardware is incapable of operating under those parameters, it's just designed to shut down so it can't be used to guide ICBMs. Commercial airplanes typically operate at half the altitude and speed which would trigger the condition.
The limits are 1,200 mph speed and 18,000 m altitude. Google tells me a 747's limits are around 615 mph and 14,000 m; Concorde's limits were closer to 1,350 mph and 18,300 m. Still, I'm sure if you're making an airliner you can get access to GPS receivers that will work even at maximum speed and altitude!
[1] http://en.wikipedia.org/w/index.php?title=CoCom&oldid=608893...