I agree about the posters strategically placed, but IMO the important thing to stress is not "don't use them" but "they must be SWITCHED OFF". Many people think that the former means not making calls or answering them but, as most of us know, the GSM is ALWAYS putting out a "ping" to detect the nearest cell and to log onto it. Many phones also adjust their transmit power according to the strength of the received cell and, at 30,000 feet of so, the cell is likely to be pretty weak and the transmitters will be "pinging" flat out.
PA announcements/posters must stress the "switched off" aspect or pax (many of them pretty technically dumb) will happily think "if I'm not making a call, that's okay".
Fly safe!