It's possible that the Demon visited Dean as a baby and Mary and John never knew (or that Mary knew and didn't tell).

You know, in addition to explaining why Mary apologizes to Sam in Home, that would put a really creepy spin on her telling Dean that angels were watching over him every night.

I don't really see the whole story becoming Dean's though, with Sam completely sidelined - I think they're always going to try to strike a balance there.