I think it must now be pretty common for 'clowns down the back' to use GPSs, as the trolley dollies dont seem to react as they once did. The first time I had one (with the antenna suctioned cupped to the window) on a Qantas 737 the following conversation ensued:
TD: Excuse me Sir, what is that?
Dr: Its a GPS!
TD: Sir, I don't think its permitted to use that on board an aircraft.
Dr: As far as I know it is, but I will turn it off if you wish. Could you please ask the Captain if its OK to use a GPS.
TD: OK, give me a few minutes.
Returns
TD: Sir, the Capt says it is OK to use a GPS while the aircraft is in the cruise but we require it to be turned off prior to descent, as we do for all portable electronic devices.
TD: Oh...and he said ONLY ONE!
Dr: OK, no worries.
Oh and for what it's worth, GPS units can interfere with other radios. Have experienced it myself. Cessna 310 with a Garmin 100 into Yalata once a month. With the GPS on it was impossible to cancel SAR, usually Perth on 6580 if I recall correctly, but as soon as the GPS was turned off HF worked fine. Just because an electronic device does not send a transmission to the antenna does not mean it's not capable of causing interference with other radios.