It really seems to me that all this regulation about personal devices is just more theatre. Or some kind of psychological strategy to emphasize to the passengers who is in control. If a cell phone or an iPod could really cause harmful interference with the avionics, they would not be allowed at all.
Here's a sampling of interference events: http://asrs.arc.nasa.gov/docs/rpsts/ped.pdf (edit: only some of them are about EMI interference)
Instruments acting up won't automatically cause a crash. But they may contribute to the accident (ref AF447)
http://www.cvel.clemson.edu/pdf/nasa-rp1374.pdf - some examples of what lack of EMI shielding can do.