I don't know why girls would want to be geeky for attention... A lot of the times I have to downplay my geekery because in a lot of people's eyes its not a good thing. I don't know how many times I've talked to guys and gone into tangents about computers, videogames, books, science, scifi, anime -- whatever, and gotten blank stares like I'm crazy. Not to mention the blank stares from other girls! So a lot of the genuine "girl geeks" or "girl gamers" tend to not want to make it so obvious. Others, like myself, just don't really care. I like what I like, the end.
I guess the thing is the 'posers' don't have the know-how to go into long conversations about those kinds of things, so that's what makes it kind of obvious. Sometimes they might see a Mario bag at Hot Topic or something and get it because its "retro" or "geek chic", which I have come to understand is more of a style than girls trying to be geeks. Like the girls who will wear band t-shirts and can't even tell you a single song they like by said band. Also, there are the hipsters who wear things for the "irony" as well. So, that triforce t-shirt? "You just wouldn't get it".
And also, "geek" is a relative term. You can be a geek about just about anything, it just tends to go by a different name (like "audiophile" or "sports fanatic"). Also, some people may think you have to be unhealthily obsessive with something to be considered a "geek", but some may think if you like Doctor Who and play any video game that's on some kind of console or PC, you're a "geek". So it really depends.