The main problem with modern mobiles is that as they get furter away from an aerial/base station, they increase their power output. If there are enough phones on and they are sufficiently far away to send out lots of power, but NOT far enough away to cut themselves off, this could interfere with a/c systems.
Many mobile companies have suggested having a base station on the aircraft itself, this would mean that the power transmitted from the mobile would be dramatically reduced.
An even neater solution being bandied about is a use for bluetooth equipped mobiles. If you have a bluetooth phone and a base station it will work as well as having a normal mobile mast on the plane, BUT a bluetooth signal has something like 10,000X less power than a normal mobile signal. Problem solved. Just got to shell out for a new phone. Again!!
Phones can have an effect on (especially) comm signals. But the danger of them is over-rated. Any properly insulated modern avionics system should NOT be affected hugely. The signals flying around the ether from a myriad of sources would have to affect the a/c too. Mobiles are easy to blame, but as yet no-one has seemingly been able to reproduce the effects supposedly caused by mobiles. Any way has nobody heard of FM immune avionics!!