"Having your cell phone on in a plane is a *BAD* thing. It ties up the cell network channels in as many as 50x the normal rate, sometimes much more depending on where you are in the plane.

On top of that, yes a *NORMALLY* operating Cell Phone won't emit any/much undue radiation, but in the event something goes wrong it could leak some broad spectrum radiation which might/would affect the avionics. And you'd never even know it was happening since there's no visual indication if the transmitter portion of the cell phone somehow chokes.

...

If everyone in an airplane who owned a phone turned it on while leaving SFO, LAX, or other large airports, it would disrupt Cell service for miles and miles. "