I personally know of one case in a glass cockpit RJ where all their screens kept blanking off and on during an approach. As they were executing a missed approach and declaring an emergency, they had their jumpseater leave the cockpit to brief the FA on what was going on. The jumpseater noticed a pax in one of the front rows talking on a cell phone and told him to hang it up. As the pax was saying "I have to hang up now" the jumpseater heard the pilots in the cockpit exclaim "there it goes again!". The jumpseater snagged the phone before it was hung up and figured out that every time you talked into the phone and it transmitted, it would blank out the cockpit screens. When you listened and the phone was in the receive mode, the screens would come back.
This incident was briefed to us during my initial training at Hawaiian back in 2001 by our FAA rep. It was a recent incident and the FAA was briefing all operators of glass cockpit aircraft. Until then, most of us thought cell phone interference with glass avionics was an old wives tale too. While rare, it does happen.
