On United, anyway, GPS units are apparently allowed. From pilot and FlyerTalker
Mark Rogers' comments on the thread
http://www.flyertalk.com/forum/Forum50/HTML/012327.html :
"GPS units are specifically allowed, while scanners are prohibited....
In the future, you might politely ask the FA to check with the captain. It might help to know that the reference is on page 8.100.7 of the FOM."