The thing that worries me is that if there is a real possibility of mobile phones interfering with critical A/C systems, then I do not believe that the present procedures, which simply involve an announcement that all mobiles should be switched off whilst engines running to be sufficiently robust. I regularly travel as SLF on Airbuses, and it is a regular occurrence for PAX to remember to switch off phones during flight, or for phones to go off. This probably represents the tip of the iceberg. I have to confess that on one occassion I have also found after the flight that I had inadvertently forgotten to turn my mobile off. So even where there is no intention to deliberately keep one's mobile on, there will be a significant failure rate in compliance.
From my own experience on smaller A/C, I usually keep my mobile on as the last line of defence backup for Radio failure. (Being over cautious on all trips other than local instructing details I also have a hand held battery operated transceiver and GPS - Call me paranoid!). I only turn the mobile off if I'm doing a "critical" instrument approach in IMC conditions. The only interference that I have experienced is the pip pip in the headset, presumably the Mobile interacting to find the relevant cell transmitter. I accept however that an A/C like the PA31 has much less critical electronic dependancy than an Airbus which is completely fly by wire.
Perhaps cabin crew should check all mobile phones when they check the boarding passes? The difficulty with this would undoubtedly be delays to boarding, and arguments with owners of "multifunctional" devices which have a flight safe mode.
One further thought, it has been announced that certain carriers will be running services which are enabled to handle mobile phones in flight. How is this consistent with the previously stated interferences to DME/FADEC? Will there be better electronic isolation of the A/C systems to facilitate this?