Why the FAA worries about your cellphone
Kind of a "Straight Dope for the Geek Guy", here's an attempted explanation for the ban on electronic devices at certain times during your flight. Still seems a little dubious to me, but what are you going to do? Some airlines are better than others. I'm never flying Iberian Airlines again with their "ningunos dispositivos electrónicos permitidos" policy: flying from Chicago to Barcelona without being able to use your laptop, MP3 player, or even noise-cancelling headphones s-u-x sux.
As suspected, the reason you can't use your cell phone has nothing to do with potential interference with navigation in the air, but instead with the ground-based infrastructure that isn't equipped to deal with a phone having line-of-sight to hundreds of cell towers at once.