Re: Never any danger
That said, radio signals have been proven to interfere with some electronic equipment - would you really want to risk it at 30,000 feet?
GSM phones can interfere with electronics, anyone who's held their phone next to a hifi system or even computer speakers will have experience that. However, it's highly unlikely to happen, given all the testing required for flight systems. If it did happen, I'd be less worried at 30,000ft than 3,000ft because the flight crew will have that much more time to do something about it. That's the reason I always assumed the 10,000ft rule, same as for seatbelts etc. Once high enough, there's a bit more chance to react.
As for flight safety, I'd say there's a huge risk to allowing mobiles to work in an aircraft, if you've got some brain-dead idiot yakking into his phone for the whole of a flight then there could be a riot when he finally irritates all those sitting nearby to the point that they snap.
"You may use your mobile on this flight, but as a courtesy to other passengers, please step outside to use it".
I always put my phone into flight mode, it saves the battery so it'll be working ten hours later when I get off the flight. If I'm planning to sleep then I might even turn it off.