If personal electronics (those carried by normal travelers; I'm not talking about portable jammers, portable high-power beacons, etc.) were at all likely to cause crashes or even incidents, a couple of them would occur every few years. On any given commercial flight, there is a high probability that someone's cell phone, ipod, laptop, GPS, etc., is accidentally left on during takeoff/landing.

To my knowledge, there has never been a documentable case of electronics interference. There's a lot of weird things that pilots have reported that end up getting blamed on electronics because there is no other easy explanation. But as you say, pilots generally aren't experts in avionics shielding and risk analysis.

And this is 2008, not 1948. It's not as if the average flight relies solely on an analog AM radio navigation beam that happens to be at the same frequency as the resonant circuit in a portable AM radio. (Not to mention that the power of the RC circuit in an average portable AM radios has shrunk quite a bit over that time. A walkman-sized radio is a lot less likely to cause interference than an older transistor radio, let alone a tube radio) The classic movie scenario of someone hanging a AM radio next to the nav aids and sending the plane off course is not really likely today. Even if it were to occur, I think ATC would get on a commercial pilot's case pretty quick over such a deviation.

My guess is that the people making the rules restricting electronics weren't electrical engineers who specialize in electromagnetics, they were "risk management" types and liability lawyers who are obsessed with having zero-risk instead of minimally-small risk. Probably the same folks who think warning labels are necessary for every commercial product and think that banning water was a sensible move by TSA.