I remember that MythBusters episode. It was shown that many phones can interfere with VOR signals and make your OBI (Omni Bearing Indicator, for the non-pilots here) go wacky. These tests were done inside of a Faraday cage with completely unshielded avionics. When they attempted the same tests inside a parked business jet, the aircraft's shielding prevented the OBI from being affected.

However, keep in mind that the OBI is what guides an aircraft along the runway's localizer and glideslope (kind of like "autoland", for the non-pilots here) and you'll realize that final approach, 200 feet above runway level, with lousy visibility is the last place you'd want to have a darned cellphone start screwing with your gauges. Would you bet your butt on your aircraft's avionics shielding in that situation, or would you rather just have people shut the cellphones off?