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.
That would be quite a good idea. I do agree with jollyrog about the GPS though, some interested PPLs may wish to have them on during the flight...purely for interests sake. One's Garmin isn't going to interfere and I'm slightly surprised the captain insisted it was off.
The avionics in aircraft you will fly in tomorrow, will not have been tested for interference from internal sources, and most likely not even for the frequencies used by most wireless equipment. If you choose to use such equipment onboard, you will in fact be conducting an unsupervised experiment.
Whipping out my 496 during the flight is not an 'unsupervised experiment' at all. While of course one should always do as you are told by the cabin crew and never argue I think if you have an aviation GPS and want to use it during flight there is no real
reason as such why you shouldn't. Mobile phones are a completely different matter though and I think even if they are ever proved safe (which I don't believe) they should be banned anyway....if only to preserve peace and quiet.