pasoundman
The idea that cellphones interfere with the avionics is
AFAIK totally unproven. However it's never been totally proven that they couldn't either.
Aircraft electrical systems are well shielded during manufacture, but the possibility of the shielding becoming degraded during airline operations/maintenance means that it is prudent to ensure that phones are not radiating during critical phases of flight.
However plans to introduce 'approved' systems for inflight cellphone use suggest that it's largely not a real concern.
There are systems being tested on some aircraft that will permit the use of mobile phones in flight. The key element of these systems is that they command all the mobiles on the aircraft to use their lowest power setting. Without this, the mobiles would step up to their highest power setting in an attempt to find a ground station and thus could get to the level where interference could become an issue - especially when there are many phones in operation. When I was in a holding pattern. I experienced autopilot fluctuations soon after advising the pax that there would be a delay of about 20 mins. Cabin Crew reported later that they had found several pax using their phones after that PA.
Apologies for the thread drift and I like many out there eagerly await the outcome of the investigation.