Spending too much time on airplanes, redux

I spend a lot of time on planes, particularly in the summer months, and every time I do, I dutifully switch off my iPod (etc.) for the takeoff and landing portions of the journey. This time, though, it occurred to me to wonder why we never get told to switch off electronic equipment during periods of turbulence when the seatbelt sign goes on (when one would assume that electrical interference would also be a problem for the pilot). Also, on this trip, I actually had my iPod on for pretty much the entire descent of the plane, with no one telling me to switch it off (I switched it off myself just out of a sense of habit, but we were almost on the ground by that point). So, is this a real safety precaution or just some kind of superstitious custom? Answers on a copy of BA stationary, please.