Seriously, if there was even an incremental risk that a powered on or active cellphone could cause a problem during a flight then they would not allow you to take them on the plane. They certainly wouldn't blindly trust people to follow the request given to turn the things off. That is just plain common sense. There is no risk at all, its simple bullshit, probably falling into the 'security theater' category.
I seem to recall John Dvorak writing an article maybe 10 or so years ago in which he theorized that the reason behind the ban was strictly accounting related. Something to do with the fact that someone on a call would switch towers too fast for calls to be tracked properly, and thusly billed properly. That makes a hell of a lot more sense to me.