I have never worn headphones while running, but I just bought an MP3 player to start using. I have decided when I am using it that I will only put one ear bud in. That way I am still aware of what is going on around me, yet still have somewhat of a distraction too. I probably wouldn't wear them in a race if it states not to wear them, but I see lots of other people doing it, so it probably isn't a problem. *_The worst they could do is tell you to put it away, right?_ *It's worth a try and if none of the officials object, then go for it!! 
Well, you could be pulled from the course and disqualified. Don't worry, it's extremely rare.
This is an on-going controversy and won't be resolved by ranting at each other here. MP3 players/earphones are banned by rule for a variety of reasons. Safety is not the first or main reason and that part of the ban is a bit trumped up by the insurers. I understand that many runners love to have their music with them. But I too have been involved in a few incidents like those already described and it can be annoying, at least. On the other hand, I've seen runners zone-out without benefit of music and earphones. I actually met one guy who wears earplugs so he doesn't have to listen to what's going on around him!
The ban is rarely enforced. But if you're going to wear them be like Kimberly, keep one earpiece in and one out. Also be aware that if you are involved in an incident, you could be DQ'ed if you're wearing earphones.
Len