As an electronics and pilot type myself, I always wonder why so many people think that rules should not apply to themselves. Especially when the topic is waaay outside their knowledge.
Not long ago, Boeing was doing testing on a WiFi installation. Turned out that on one of their airliner models, if enough passengers turned on WiFi, the pilot's LCD instrument panels would just go blank. Not enough shielding, because they were never designed for such a situation. Easily fixed, but it points out that each aircraft model needs individual (and rigorous) testing.
Want to get really worried? Read some of the anonymous NASA safety reports that pilots file on this topic.
Here's a PDF of some.
Everything from Navigation radios screwing up, to autopilots kicking off or diving the plane at the last second, to radio interference - imagine the sound of dozens of GSM phone buzz. There also seem to be a lot of false TCAS collision alert messages, causing the pilots to take evasive action for no reason. I remember one report where that happened during the landing phase and almost caused a crash... all because some idiot passenger thought she could sneak in a phone call.
There's also so many reports of batteries and power outlets catching fire. (It's just a matter of time before someone's iPod stashed in the luggage compartment brings a plane down. Falling out of the sky on fire is one of the worst ways to die that I can think of. At that point, expect a much stronger ban to appear.)
At the least, interference can cause unnecessary distractions for the pilots. Only the most self-centered people think that's okay.
Folks, it's just not worth the risk yet. As more and more modern airliners come online, I think the chances of interference will lessen a bit.