This is only going to get trickier, as gadgets get more complicated. Any digital device (and many analogue ones) emit low level radio signals during normal operations - hold an AM transistor radio next to your laptop - and as more of them get radio capabilities which may not be explicitly clear to the user it's going to be trickier policing them in flight.
GPS receivers should be safer than most. GPS signals are very weak, so the designers will have taken care to shield the incidental radiation from the electronics. That's just as well, really: I was talking to some engineers from Broadcom, a chip company, and they said that GPS is going to be a big growth area with navigation capabilities finding their way into all sorts of consumer electronics. With more seatback entertainment systems sporting moving maps, it's no longer quite as fun to use a handheld GPS during flight as it was, but the chances of your MP3 player, wristwatch or laptop having GPS can only get higher.
If there is a problem - and despite anecdotal evidence, I don't think there is - then the only solution I can think of is the worldwide adoption of a flight-safe certification scheme for portable electronics, with a recognisable logo on the case that can be shown to cabin crew, alongside a similar logo on the display that shows when flight-safe mode is enabled, if that's an option. It could be run like the CE, FCC or other approval procedures, where manufacturers self-certify and cop the blame if they lie.
Personally, I'd think this would good thing: I'd love to be able to use a radio receiver during flight to catch up with the World Service on HF, and while the ban on those is sensible given the way radios work it's quite possible to design one that doesn't cause problems.
R