A couple of weeks back while on a training exercise in my 2-seat thing with a fan on the front, my instructor had his cellphone sat on top of the instrument housing.
While practicing tracking a course I heard a loud buzzing through the I/C and saw a deflection on the magnetic compass or 6 or 7 degrees and the GPS display became distorted. Not two or three seconds later the phone beeped with the tell tale sign of a text message then everything returned to normal. I believe the phone was polled by the ground station and in response belted out a high power Tx that caused the problems.
Of course I can't speak on behalf of you parrafin powered boys, but my cellphone is going to stay switched off as soon as I walk out onto the apron. It's not worth risking anything else as far as I'm concerned.
Aq