Today I read an article written by Dave McGillivray, the race director for the Boston Marathon. He was talking about the use of iPods specifically, but what he meant was MP3 players in general. However, he does point out that the iPod was the biggest brand he consistently saw.
The rub is whether headphones are good and healthy for runners. As a race director of a USATF sanctioned event, he has to lay down the law and say that no headphones are allowed. This is for insurance purposes, but also has social reasons. He does a good job of pointing out the reasons why one should not wear headphones while running. I agree with many of his reasons.
However, I run almost all of my training runs with headphones. If I am fortunate enough to get someone to run with me, then I will not take my player with me at all. Or, sometimes during speed workouts I will ditch the player. Those are times that I am focusing on the event and not the podcasts. But at all other times I wear my headphones during training.
I used to wear them during races too. After my 3rd race or so, I gave up on them. I wanted to experience the atmosphere as a whole and do my best while running. I really enjoy chatting with other runners. While I am not a competitive runner, I do try to push myself at races.