There are well documented cases of aircraft systems being affected by use of cellphones, typically fire warnings.
Most of these aircraft systems will have been designed before cellphones were invented (how long has the B747 been in service?) so it's not a case of designing them so they resist it.
Standards vary around the world with different frequencies and power outputs being used in different countries so you would in any case be trying to hit a moving target all the time, akin to trying to nail jelly to the wall.
Since there is no imperative to use a cellphone while airborne and since the network operators don't want it done then banning their use in flight seems eminently sensible IMHO.
I used to be in Royal Signals and we were operating a 350watt HF set. My radio operator had rigged up a wire aerial but it was not connected and so not in use, he was using a vertical whip instead. When he reached out to move the (disconnected) wire the shock blew him off the top of a one ton truck. Don't underestimate the amount of power that can be picked up by a pice of wire that just happens to be the right length to make it resonate at the frequency in use.
I also remember many years ago the story of someone living near the Crystal Palace TV transmitter who rigged up a chickenwire contraption in his back garden and gathered enough power to light his house, at the same time blacking out TV reception over a large chunk of Kent!
Mike