An interesting anecdote regarding GPS. One system I used about three years ago had two GPS receivers and checked one against the other. One day, receiver 'A' reported no satellites were visible, while receiver 'B' showed a sky-full, albeit it slightly reduced signal level compared to normal. We tried changing the antenna on receiver 'A' for a known-good unit, but still couldn't see any satellites. We scratched our heads for a bit until I suggested using the antenna we'd just removed on receiver 'B' instead. That one change brought BOTH receivers back to life, with strong signals from all satellites. A lab investigation showed that the low-noise amp in the antenna which was originally on receiver 'B', the one that appeared to be working, had developed a fault and was acting as an oscillator which was blocking the other antenna, located some 20m away.
Now imagine your personal GPS has a similar fault. You're sitting in the cabin of an airliner satisfying your curiosity by looking at your moving map, oblivious of the fact that your kit is interfering with the aircraft's GPS. Not a problem in a light aircraft since generally the pilot knows full well that someone in the back seat is using a GPS and can ask them to turn it off if they have the slightest suspicion of a problem. How about the Captain flying 200+ passengers? He or she has absolutely no idea why his GPS doesn't work any more - and if in IMC, they may not even be aware that it's not working.
In general, during critical phases of flight (loosely those which occur within 10,000' of the ground for an airliner), I make darned sure that all of my personal electronic devices are turned off if they can be. I do draw the line at my wrist watch, and if I had one, I'd probably leave my pacemaker on too
It appears from the accident rate that the risk is low even for devices which are designed to transmit (e.g. the infamous cellphone), although there is a growing body of evidence that non-critical system interference does occur fairly regularly. But why take that risk? Is a text message really worth 200 lives?