I have mixed feelings on this.
I used to do a lot of running with an mp3 player....
Personally though, as I never race with one, I'd sooner do all my training closer to 'race conditions'. The other thing is you become a lot more in tune with what your body is doing... running too hard, injuries etc. if you are running with no music. It's a good distraction from the pain of hard exercise.... haha.. but sometimes you need to be 'feeling' and 'hearing' what your body is doing and the effects of the run.