Originally Posted by crowe
Two points on interference.
I've travelled with my cellphone in the trunk of a large car, at least as far from the car radio as you can be from the cockpit of a single aisle airliner. I can still hear significant interference on the radio as the phone searches for a signal - and I don't want that to occur at the moment ATC tell the pilot "immediate hard left for collision avoidance" or something similar.
Secondly as was mentioned above not all of the items that could be influenced are in the cockpit, there are looms of wiring and instrumentation throughout the plane.
Aviation has developed into such a safe method of transport by overengineering safety to a very high degree - this is another example of that.
Although I agree with your second point in theory (yes there can be interference in close proximity), airlines will soon start offering micro-cell service on aircraft with little or non of this concern.
As for your example of interference while the cell phone is in the trunk. The interference was more likely actually between your cell phone in the trunk and your rear speakers (which are also mounted in the trunk of the car).
I'm not saying that if you put your cell phone on top of the GPS unit that it won't interfere, it most likely actually will (try putting any GSM or TDMA phone by a CRT beam monitor for an example), but the effective interference decreases exponentially as you move as little as one foot from the equipment.
Perhaps the rule in fact protects us by keeping pilots from keeping their phones on.... whether intentionally or not.