I searched American, Delta, United, Northwest, Continental, JetBlue, ATA, Southwest, & America West's websites, and only 2 of these airlines had anything at all to say about GPS units, (United & Continental had lists of devices, but GPS units were not on either the approved or non-approved lists)
From Delta's website:
You may not operate the following devices when the airplane is at the gate and the boarding door is closed. This restriction also applies during taxi, takeoff, initial climb, approach, or landing.
Personal computers
Personal computer peripheral devices
Personal digital assistants, except when taxiing to the gate
Personal computer games, except when taxiing to the gate
Compact disc players
Digital cassette tape player/recorders
Video recorder/playback systems
Calculators
GPS (global positioning satellite) systems, except when taxiing to the gate
Due to safety concerns, flight crew personnel may, at any time, ask you to turn off a personal electronic device.
From Southwest's website:
When indicated by Flight Attendant announcements, during the cruise phase of flight these devices may be operated: DVD, CD, and tape players, personal digital assistants (PDAs or hand-held computers), electronic calculators, hand held electronic games, electronic cameras, Global Positioning Satellite ( GPS ) receivers, and devices that compose e-mail or text messages without transmitting or receiving.