Technology isn't always the answer but I used to imagine a system which when activated, such as in a cinema before the movie starts or on an aeroplane, would cause a phone to respond with a beep if it was turned on and not made silent. It possibly wouldn't stop someone who wanted to fly around in silent mode but it'd make the honest ones aware that they'd forgotten to turn off their phones.
It'd take some collaboration on the part of phone manufacturers but surely bluetooth phones would just need a software upgrade.
Just a half-baked idea,
~FRQ CB