I don't believe the iPod and other mp3 players are to be turned off due to "interference" with the airplane's systems. Rather, I believe that it is enforced for the fact that if something goes wrong during takeoff or landing, it is easier to alert the passengers what is wrong and give them instructions, especially emergency instructions including evacuation tactics. If someone is blaring their mp3 player, ignoring the usual dings and announcements that are common to takeoff and landing (such as the we are approaching such and such a city, the weather is blah blah blah...) wouldn't hear a crucial message, it becomes a safety hazard. My guess is that it is more of a safety/liability issue to not be able to hear the FA or Pilot's announcements during the first and last segments of the flight. Just my opinion...