Flying from Detroit to Louisville, Ky this weekend, I asked the flight attendant about using my GPSr at altitude (I made sure to say "at altitude") when we were boarding. I was still within earshot and heard the captain say "Sure!"
Once electronic devices were allowed, I looked out my window at Toledo; Dayton and Cincinnati were out the other window . Speed stayed around 460mph, elevation around 28700ft. Before we hit cloud cover, I saw the airport symbol on the display, looked out the window and saw the small town airport (one runway for private planes). The accuracy bounced between 50 and 90 feet with my window seat and tray table. I wish I could have used it during take-off/ascent and descent/landing, just to look at the numbers.
This was too cool. I suggest everyone to take their unit when they fly, but be sure to get permission.