This is something I'm watching quite closely, as I write a lot about portable electronic gizmos and associated stuff. Some phone manufacturers are putting 'flight mode' options on their kit, which lets you use your mobile with the transmitter disabled -- a good idea in theory, but it must make the job of the cabin crew impossible as how on earth do you tell whether a phone has that mode or whether it's enabled? On the other hand, we're getting towards the time when *everything* will have some sort of transmitter in it... My phone has a calendar, alarm, address book and other nonsense inside which I might well want to use during a flight, and I've seen prototypes of wristwatches with Bluetooth -- that's the new portable wireless network that will end up in anything with a chip.
I can't see the problem going away.
I'd start thinking about putting automatic signal detectors in each seat or, perhaps cheaper, in the overheads. You'd need to have some sort of filter to avoid them triggering when the a/c's own radios were in use and in high RF areas near radars and so on. But I'd guess with the hundreds of thousands of seats out there, the market would be big enough to drive the cost of such a device down to a feasible level.
R