Always fascinating to hear the opinions of the armchair experts.
There is evidence to suggest mobile phones may have interfered with aircraft systems, and if you search the CAA aeronautical information circulars on this subject, you can read them for yourselves. One point you might want to consider is that an aircraft structure has a vast array of transmitting and receiving aerials built into it. The architecture of the aircraft is such that the placement of these devices is designed to prevent cross interference between the receivers/transmitters and each other and with other sensitive aircraft systems. It doesn't take a huge amount of common sense (even from the most ardent conspiracy seekers) to realise that the introduction of additional mobile random tranceivers into this environment is likely to prove disprutive on many levels.
From my own experience the sound of loud mobile phone search signals in my headset whilst taxying at a busy airport is very distracting. It is therefore unacceptable. In addition is very unlikely that a mobile phone ( cellphone) will successfully aquire a signal in cruise flight despite increasing its output strength in an attempt to try. Potential disruption to communications or other systems ( however small) is most certainly undesirable during the climb, descent and approach phases of flight. During these phases most airlines will also restrict the use of other small electrical devices ( computers/CD players etc).
As regards sept 11th, I think you will find that many domestic airliners in the US are fitted with seatback flight phones. It was from these that calls were made. These phones are built into the electronic architecture of the aircraft and are therefore certified for use during flight. That notwithstanding such a catastophic event lends precious little arguement to the suggestion that mobile phones do not cause interference.
On a positive note, there seems to be some serious development of technology that may permit the use ( albeit perhaps limited ) of mobile phones at some point in the future. It would of course be unacceptable for a passenger to be using any device that distracts from the ability of the user or those around him to properly monitor such things as safety briefings.Any advances in the technology will therefore have to satisfy the regulatory authorities in all respects.