I'm definitely in the "don't do it during bike races but why the heck not during running races" camp. Those who say we shouldn't do it because the Olympians don't do it are being naive, in my opinion. If Olympians are your example, then I assume you think steroids are okay to use? Obviously I am not saying that all Olympians are steroid users, but recently there has been an alarming number of them, especially in track and field. Not necessarily the greatest example.
And why are we saying that you are totally responsible for knowing what is going on behind you? Last I heard, whether you're on a bike or in a car, it is primarily the responsibility of the person in back to avoid hitting the person in front. You rear end another car, YOU get the ticket, almost always. Granted, the person in front should take reasonable precautions, but the person in back obviously has a better view of the whole situation and therefore has a better chance of avoiding accidents.
As a runner (especially a slow one) I stay to the right and check around me if I'm going to take a walk break or something. That's just what you're supposed to do, and it makes no difference if I'm wearing headphones or not. I run on paved paths that are shared with cyclists. I often can't hear them coming, regardless of whether my music is on or not. A nice "on your left" given with enough warning to let me make sure I'm out of the way is great, but sadly doesn't happen all the time.
I am a midpacker at best, and thus harbor no illusions that I might even place in my age group, much less overall. I'm out there because it is fun for me, great exercise, and I'm just a bit of a ***** for finisher's medallions and event t-shirts.

I'm happy if I can pass a few people and they don't pass me back right before the finish line. Someone who can finish in the top 5 is WAY beyond the point where headphones have anything to do with it. More power to them, I say.
Amy