I seriously doubt that a cell phone would cause something to go majorly wrong in a plane.
The magnetic fields in and around a cellphone can however affect the compass but you have to be in quite close range to it. Put a cellphone close to the compass and see what happens, just don't tell the engineers that you did it!
Apart from that I don't think it has much affect, but just think about it... If you fly the new A380 and you've got how many hundred souls in the back chatting away on the cellphones, it's gonna sound like you're listening to morse code on the radio and it's gonna annoy the living hell out of us pilots!
I have also heard that using a cellphone that high up, if you actually get a signal, you bugger up the signal for the users on the ground, don't know how true it is though...