I first noticed this years ago when a new, young guy who worked at the radio station would never, ever take his headphones off. I think he did something to do with the web site. I'm not sure, because I never actually heard him speak. I'd approach his desk with a question, and he'd look up and look at me and nod. I wasn't ever quite sure if he'd heard me or not, so I'd ask "can you hear me?" and he'd nod again.
A couple years later I was working behind the counter of my frozen yogurt store (I was fired from the radio job. Headphone guy was not) and another young man came in with sweats on and earphones jammed into both ears. I could actually hear the rap music that was playing, it was so loud. He was having trouble understanding me, so he kindly took out ONE of his earphones. He was not hired.
Don't get me wrong, I like it when you listen to the radio with your headphones on, and it's made trips to the beach much more enjoyable because we don't have to listen to the awful music those other people are listening to. But jeez, can't we all agree to take the darn things off when somebody's trying to talk to us. We've got a few of those people around our building, and they might as well put up a sign that says "Don't Talk To Me! I'm Listening To Something More Interesting Than You!"