I recently traveled to Philadelphia from Little Rock, via Memphis and Detroit on Northwest. I stored my GPSr in my camera bag when going thru TSA. No problem. On boarding I stopped by the pilots cabin and asked if I could use my GPSr in flight. No one objected. Just obey the "electronic device announcements" The captains even asked me when I deplaned if it worked. I was by the window and it worked perfectly. The only problem was the altimiter reading was false due to the artifical cabin pressure. As a side note the official TSA website says "all electronic devices are allowed". Its really interesting to see what "roads" the plane is flying over.
