I'm a geek on many counts, but I don't usually say anything about it unless I get a signal that the person I'm talking to is also geeky in some way. Usually, though, it's a good idea to bring it up at some point early on, in my opinion. I still know enough about pop culture that I can talk about the idiotic celebrities that most the idiotic masses worship without tipping them off to much.

If it's a girl I plan to have come over at some point, eventually they're going to see my walls covered in anime posters, my shelf of anime figures, stacks of comic books, drawers filled with manga, TV with videogame systems all over the floor, etc. I've got so much geek stuff in my room it would be literally impossible to just hide it before a girl comes over, so it's really best for me to eventually tell her about it. If she can't deal with it, better now than later.