Up to now, everything in the avion industry has to be certified free from interference with other onboard systems : it includes avionics from the flight decks, flight controls systems, engine control systems, also in the military all the warheads with various internal seekers and propellors (which are not only for jet fighters but also for big stuff like Nimrod, Orions, Atlantic..) Believe me, it's a long process, very expensive but also quite reliable (what do you think people do for years in the various government or industry test centers?)
So after certification things are rather safe (or one could contempt certification, to replace it with what?)
And even with that, given the life-span of the aircraft of systems, funny things could occur, which are/could be explained (but once again, in aerospace science, you need to be sure that the same causes will entail the same effects, so who is going to pay for the test program?)
Some funny things I heard about in my little career:
Old Jaguar fighters in the 80 shivering when flying close to VLF antennas (the one wich are 1000 ft high, to communicate with submarines)
A safety bolt from the rescue winch of a Lynx helicopter being (allegedly?)triggered (oops..) by a powerful radar system of a new frigate
So coming back to onboard cell phones, with all their different and changing characteristics, to obtain certification with somebody (a/c manufacturer? phone company?)signing (and paying for it) for it... good luck !!