I have a birth mark on my neck and on my left arm. To this day, grown men and women I don't even know get this really concerned look on their face, come right up to me and ask "What's wrong with your neck?" or "What happened to you?" It's happened so much in the past that I just take it in stride, but it strikes me as kinda rude. I mean, if I see some guy with one leg in a wheelchair I don't say "What happened to you?" Though that might lead to a good story.
I think my birthmark is a part of me more than any tattoo could ever be.