There are some studies that have been done by the FAA, Transport Canada and others that do show some interference between cell phones, electronics and avionics equipment. However the number of conditions that have to be perfect for the interference to happen on modern aircraft is are so high that it will never happen. Newer airliners also have better shielding around the avionics systems.
The real problem with cell phones is a cell phone in the air could easily overload the cell tower network on the ground. Each phone is only supposed to be "visible" to at most two towers when transitioning from one tower's coverage area (or cell, hence cell phones) to another. Due to buildings, hills, and other factors, this is easy to control on the ground, but in a plane, you could easily have a clear path to several towers. The results can show up to both users on the ground and in the air as lost or busy signals. Until the cell companies bring out new networks that can handle this situation, they will be banned from use on planes. As September 11th shows though they can work, but if everyone tried to, the cell networks would collapse.
Of course the airlines are only too happy to provide you with their own phone system to use and pay for.
Boeing's Connexion system does allow wireless access on the plane and has been installed by Lufthansa. There was something in the news recently about this I think.
goates