See this FCC link for the mainreason they were banned - COMMERCIAL!
http://www.fcc.gov/cgb/consumerfacts/cellonplanes.pdf
Extract
"FCC rules currently ban cell phone use after a plane has taken off because of
potential interference to cellular phone networks on the ground.In addition, the Federal Aviation Administration (FAA) has rules prohibiting in-flight cell phone use because of potential interference to navigation and aircraft systems."
At least the FCC is honest!
I remember years ago Noel Edmonds (UK TV Personality) flew his heli from Nth Scotland to Southern England while BBC TV Tomorrow's World tracked him by the cell activity. The simple fact is that when airborne, the phone locks on to more than one or two stations which can (or at least in earlier days
could ) lock up precious lines.
Nowadays, I reckon the main safety hazard is n the ground, taxying, phones all coming alive, ATC spot a conflict, call "XYZ123 hold burpp burpp... "
Result maybe an accident.