On a trip to London Gatwick prior to 9/11 happening and all the enhanced security, I fired up out my Etrex Vista midflight. It was very tough to get satellite lock, and I had a window seat. I managed to get a few signals and get poor accuracy, but enough to start calculating speeds. It was very cool until a flight attendent came to me and kindly instructed me to turn my cell phone off.
My explanation of the fact that it is not a cell phone but a GPS only made the flight attendent more uneasy. I tried to explain that it is passive in its use, and does not transmit anything and that the plane uses the same technology itself, but he wasn't buying any of that.
There were no threats to take it away from me - just to simply turn it off, so I turned it off and put it away. I was very obvious with my use of it since I had to get it as close to the window in order to get some type of signal, so it didn't make any passengers uneasy about its use, but it was also prior to 9/11. There is a lot more sensitivity and awareness to strange goings on that would make me feel more uncomfortable about using it now.
Also, as much as I wanted to record the takeoff and landing on the GPS's log, I did respect the not electronic devices rule during landing and takeoff.