Well, I can definitely say, without a shadow of a doubt, that a mobile phone left switched on CAN have an effect on navigational equipment, namely the NDB. This is in a light piston twin, but it proves that they can cause emissions in a wave band that might interfere with navigational equipment.
On an instrument training flight, I noticed that the NDB reading seemed to be unreliable. I would normally have suspected some kind of electrical storm activity, except that the conditions outside the screens were CAVOK with no storm activity forecast or reported.
I noticed that the interference was transitory, but was causing a swing to the exact same point on the dial (towards the rear of the aircraft), no matter what direction the aircraft was doing. This told me the interference was on-board the aircraft.
This was the only indication of a problem, no other instruments were showing spurios or unexpected readings of any kind so I continued the flight and carried out an ILS with the screens down for safety. This was perfectly safe due to the good VMC.
On taxying in we investigated the aircraft, and found that a mobile phone had been dropped behind the rear seats, and the owners attempts to find it by ringing it had been the cause of the interference.
So can mobiles interfere with the equipment, undoubtedly. Is this likely to be a risk?
Well, imagine a descent into a field way out in the middle of nowhere, the only approach is an NDB approach, and you have obstructions on either side of the approach track. You are flying at minima, so no visual clues, and the NDB needle is dancing around. Are you likely to trust the NDB? At the very least you're likely to get a go around and probably a diversion!