Count me with the "Headphones mean leave me be" crowd. I don't wear them often, but when I do, it's because I don't want to talk or be disturbed. I think since they're devices that are specifically designed to block out sound, it's fair to assume that the person wearing them isn't actively looking for conversation and probably doesn't expect to be part of one.
I thought they were specifically designed to keep *your* sound from bothering other people.
For me, it's a bit of both. But I wear noise canceling headphones due to my range of hearing being greater than what is considered normal. So, I'll probably still hear you but I am going to ask you to repeat your question so I know exactly what I'm answering.
However, if I have my headphones on, it's to make sure that I don't have a public meltdown. I do not tolerate people's voices very well due to said hearing. On any given day, everyone can sound like the volume is turned up to 11. However, if I'm out during one of those days, I keep my headphones on and I don't make eye contact with people. Am I rude? Maybe, but it's a bit of self preservation and to keep myself from making a scene and/or having to deal with a migraine later because someone asked a question.