The instructions in the Flight Operations manual used by the crew are very clear. GPS's are allowed. But under no circumstance may a suction cup or any other device be attached to the window or hull of the aircraft. But a GPS, by itself, is perfectly allowed.

I'm a GM and only use a GPS if I am upgraded, and that hasn't happened on flights since May. But at the time, I had a flight attendant tell me I could not use it. I asked her to check with the captain and verify there use was still permitted per the FOM. It was. (She apologized, by the way, but I had already put it away, obeying a lawful order).

I have used a GPS on Delta for about 9 years now.

Also, while Joe's site (linked to above) is good, it's not completely accurate. QF, for example, forbids GPS, and this is posted in their onboard magazine as well as announcements.