A mate was flying as a pax recently when the stewardess came down the aisle and asked if he had his mobile on, as the cockpit was experiencing some interference. He checked and found it was off, but it turned out the woman next to him had actually left hers on.
I know many pilots fly with their mobiles and use them on the flightdeck, but they definitely do make some difference.
Does it depend on where in the a/c the phone is on? In relation to systems/wiring etc.