This is all completely pointless anyway when you consider most flightdeck crew leave their phones switched on in their flight bags all day with little/no effect.
The point is that there have been several reports of mobile phones interfering with navigation equipment in flight. No one has pin-pointed the exact reason so the ban is a safety precaution. We're talking about an aircraft here and the safety of the people within.
My ancient car has no iPod docking station so I have a tape cassette emulator that slots into the cassette player. While I am driving along listening to the music I often get the phone 'searching' for a connection. The 'throbbing' noise picked up by the cassette player - a simple magnetically coupled audio frequency device - is deafening. There is no doubt in my Avionics Engineer mind that mobile phones pose a serious hazard to aerial navigation and the ban on their use in and around aircraft is fully justified.