The point a lot of you miss is that there most certainly will be dozens of mobile phones on board. So I'm afraid it's one rule for one, one rule for all.
So derekl, your mobile may not pose a risk at a given time, but if the other 100 cell phones sharing the same metal tube with you join the party, you sure as hell are going to multiply the risk of, for example, the flight directors going off in different directions.