There was an interesting article in last month's IEEE (US Institue of Electrical and Electronic Engineers) Spectrum magazine that summarized this whole issue. There have been numerous documented cases of auto pilots etc steering planes in completely the wrong direction. In several of these cases, the captain has managed to locate a specific mobile phone that was the cause of the error. Laptops with 802.11 and Bluetooth have also been seen to cause problems.
As part of the research they put some monitoring equipment on domestic US flights and the number of instances of people deliberately using phones was very high and IIRC the summary of the article was that it was only a matter of time . . . .