The USAT (USA Triathlon) rules also forbid headphones, and that rule is almost always strictly enforced at triathlons. Some race directors will confiscate your headphones or even disqualify you on the first offense. However, triathlons are much more competitive, and are much more complicated than your typical weekend fun run.

Elite runners are usually banned from using headphones, because they could be used with cellphones to get information from coaches or spectators that would give them an unfair advantage over other elite runners.

Most  fun-runs and even the larger races are filled with people listening to their music players. Whatever gets people motivated and out there exercising is a good thing. Listening to music while running is not a safety concern, and no study has proven otherwise. If your music is so loud you can't hear anyone next to you, then its probably too loud. Deaf people run all the time, and people who can hear fine can sometimes zone out, especially in an intense race. If you train with music, run the race with music. It will keep you going, especially on the long runs. I've run my best races this year listening to mine.

I ran half marathon last october and wore my mp3, but I didn't know about that rule,

I think I won't use it anymore in the follow race.  I don't why the rule but, I have to

respect it.

Nice point of discussion