Use a GPSR in a plane? No, never, very dangerous, that will cause the plane to crash right away.
Seriously - I cannot see any technical reason for not using a GPSR in an aircraft. I suppose that if you put the GPSR right beside, for example, the aircraft's own GPS antenna, it might cause problems - normally GPS antennas on aircraft need to be 3 feet apart - but it would be pretty difficult to get that close to the antenna on any large passenger aircraft.
Many cabin crew are not familiar with what GPSR's are, so, they lump them into the general category of "if it's not on the permitted list, then don't use it". My experience as a passenger has been that if the cabin crew know what a GPSR is, they generally let you use it. If they don't know what it is, they generally request that you not use it.
It's difficult to get reliable signal reception in an aircraft unless you put the antenna right up to the window. It's best to hold the antenna at the very bottom of the window. Swivel-type antennas, such as the SP III or 176/276/296, work best. Hold them at the bottom of the window, point them out at about a 45 degree angle off vertical.
PanEuropean
(still too busy to get to all the other messages but hopefully will be back soon).
PS: Here's a real smile for all of you - it proves that it is a very small world after all: The photo was taken in Canada this summer when I was training some new pilots. The regular pilot of the plane in the picture is none other than Twotter, the person who made the post just above this one!