Mythbusters had an episode a while back where they explored cell phone interference on avionics. They had an airplane cockpit and a radio that simulated cell phone frequencies but with adjustable power levels. IIRC there was no effect on the avionics when it was at the power level of 1 or 2 phones but when they dialed it up to higher levels to simulate many phones in use it had a noticeable effect on distorting some of the instruments.