The MP3 player debate seems mainly to rest on two issues: Liability/Safety and Unfair Advantage.
Liability/Safety: I just returned from the Ukraine where I took a holiday. Interesting country, and very different in some ways from good 'ol USA. Relating to our issue here, if Yuri in Ukraine walks down the street, trips and falls on a bit of sidewalk that is incorrectly protruding, he cannot sue the city or the land owner. It is Yuri's fault for not watching where he put his feet! If a runner fails prudent caution and runs out in front of semi, whose fault is that? The race officials? Or the city or property owner? I think not! We spent too much time in this country being responsible for other people's stupidity and trying to protect them from themselves. On the other hand, if Yuri gets drunk and smashes my car with his, Yuri may go to jail for that, but I will have to repair my own car. How fair is that? NOT!!! There are two sides to the liability/safety issue, but people need to learn to be more self responsible.
Unfair Advantage: It may be that the elite runners up front of the race competing for prize and status should not wear earphone to prevent them from receiving information which could give them an advantage over others. But what about the rest of us? The last time I ran a half marathon I was so blazing fast that five full marathon runners finished ahead of me. Exactly what kind of advantage was I going to receive???
And of course there are other issues and more possible technology to be had and where does it all end...if it will end? My position: Fair competition needs to be protected, but beyond that a runner needs to do what will enhance the journey called running.