In my opinion, the higher the number of people out there in traffic (streets AND paths) who wear earphones while jogging, biking, roller blading, or walking, the more accidents we are going to be seeing.
Yes, listening to music IS enjoyable while biking or jogging....but even aside from the hearing impairment, it DOES pull some of your concentration away from what's happening around you on the road at that instant, just like having your mind wandering. The combination of bikers listening to headphones and drivers talking on cell phones is incredibly dangerous.
In my humble opinion, music is fine when you're exercising in the gym- it's a safe environment where collisions with other exercisers are seldom fatal.