Those of us who actually work in aviation haven't got a clue about anything. The only reason we tell you to switch everything off is to make you as uncomfortable as humanly possible! We thoroughly enjoy imposing rules and restrictions that make no sense, just to ensure your travel experience will make a North Korean death camp seem like heaven!
On a more serious note:
In aviation, absolutely everything we do revolves around safety! There are many reasons why you have to switch of all electronic equipment during takeoff and landings, some of them are obvious, some a little less so.
Contrary to the popular stories, even a single ipod can emit electromagnetic waves that are higher than what is deemed allowable on an aircraft. Keep in mind that everything in aviation has a large safety margin to ensure the safe operation of the aircraft, meaning there is a big difference between whats allowable and what is potentially hazardous. The emissions of a single ipod isn't going to cause any noticeable interference, but when you multiply it by 300+, it could affect aircraft systems.
Your ipod, and most electronics sold in the west are made to stringent quality criteria, but there are many substandard devices in circulation. New gadgets are released on a daily basis, and there is no way that any flight attendant can keep up with all of them. Rather than risk a dodgy device interfering with systems, it is safer to ensure all are switched off, especially during the most critical phases of flight, namely takeoff and landings.
The VHF radios used by pilots to communicate with Air Traffic Control are susceptible to interference from mobile phones, making that unmistakable noise you get when you put your mobile phone next to your stereo. While this isn't so much a problem during the cruise phase, it can become problematic during departure and arrival as the pilots are in almost constant communication with ATC.
All vital aircraft systems have several layers of redundancy, meaning if one system fails, another is ready and waiting to take over its functions. Some of the more low tech backup systems are not as well shielded as the primary systems. If for any reason an important system should fail, the backup systems must be ready to take over their function immediately. If something is to go wrong during takeoff or landing, it tends to go wrong very quickly. This is why we ask you to switch off your equipment ahead of time, to ensure that it is off in case we need it to be off.
During the cruise phase, when the aircraft is at cruising altitude, you are free to use your electronic devices. If something was to happen at this stage of the flight, the crew will have plenty of time to instruct you in what to do. More and more airlines are introducing wi-fi, and some have even introduced systems that allow you to use your own mobile phone to make and receive calls and text messages.
As I mentioned above, when things go wrong during takeoff, climb, approach or landing, they tend to go wrong very very quickly!
Imagine that both engines stopped working on final approach, and you only have 30 seconds before impact. Then imagine 400 passengers all immersed into their own little worlds, listening to music on their mp3 players, playing games on their ipads, etc… Nobody will hear that the engines have stopped working, and there won't be enough time for the crew to walk down the aisle and individually rip the headphones out of your ears to tell you that you need to adopt the brace position as an impact is imminent. Oblivious to the danger, many passengers will sustain serious injuries that could easily have been avoided by adopting the brace position.
Once the aircraft comes to a complete stop, the clock starts ticking. The crew only has 90 seconds to get everybody off the aircraft before it is likely to explode. All the injured passengers will hamper the evacuation, and before you know it, time is up and hundreds of passengers who could have survived has burned to death.
These kinds of situations are extremely rare. The chances of being involved in an aircraft accident are miniscule, but for those of us who work in aviation, these kinds of situations are always on our minds. Air travel remains the safest mode of transport known to mankind because we always try to stay a step ahead. We always imagine the worst case scenarios, and how we will deal with them, so that you don't have to. If the proverbial turd hits the fan on one of your flights, all you have to do is obey our instructions.
If you can't survive without your ipod for a total of about 20 minutes on each flight, maybe flying isn't for you.