Before September 11 I had taken a flight aboard Southwest. There were no problems getting on with it. The in flight magazine indicated that a GPSr was allowed in flight, but not during takeoff and landing. I had fun using it and logged a couple of interesting waypoints along the way. As far as these post 9/11 days, I am not sure if they have changed their rules for use.
I never get lost!
I simply investigate alternate destinations!