There is zero chance of emissions from mobile devices affecting an aircraft. However it is obvious from the responses here that a lot of people still believe it is. We all watch too many Movies. There are lots of shows that show what really happens. Mythbusters does a fairly good job most of the time - big exception is the Lie Detector episode (the Lie detector machine is bogus - the operator is the one deciding if you are lying).
One reason that you are not supposed to use an iPod (and it was walkmen before that) before takeoff is that they want you to pay attention to the warning / instructional videos and air safety announcements. Originally the emissions from mobile devices were feared to interfere with communications but that has been proven long ago to be a baseless fear.
Do you think 300 mobile phones in an aircraft which is on the ground, then taxi-ing for takeoff, then taking off and so on are emitting as much interference as the signals from the mobile towers and all the other sources of radiation and radio waves that are outside the aircraft? How about all the TV screen and entertainment systems in modern aircraft? Your mobile / tablet / MP3 player will not even register a blip. I bet less than half the people on a plane even put their phones into aircraft mode (or even know how to).