My understanding of the ban is not made by the FAA, it is mandated by the FCC. They do not interfere with the aircraft, but with the cell phone system.

Again, as I understand it, the system is designed so that a hand phone can "see" and "register" a limited number of towers at a time. When you get the phone in the air it can "see" and "register" with *many* more towers, causing problems with the capacity planning model of the cell phone system. There also may be issues with a cell being able to register with more than the designed number of towers.