I have that same t-shirt.
I'm extremely out. It would be impossible for me not to be out as a geek at work, since my boss is my equal in that regard - his office walls have Trek posters on them, and he has a couple of Trek coffee-table books on his shelves. Plus, he reports to me on two convention committees, and... he posts here.

(I don't believe he's ever posted in Misc., though.)
When I was in high school, I was so out that a friend of mine had a t-shirt made up as a gift to me that said, "I'm a Trekkie - what's your excuse?" I wore it at least once a week.
I also wear a geeky t-shirt to my bowling league every week - whether it's something amusing like the one in
thestrangequark's photo:
or something mathematical:
front:
back:
Sometimes I like a little innuendo with my math:
or something with a bad pun on it. I don't have any images of those ones, but one says "Space, the vinyl frontier" with a cartoon drawing of the Enterprise, with the saucer replaced by a 33-1/3 rpm record; another says "At the Zen pizzeria" and has a cartoon of a guy standing at the counter saying, "Make me One with everything". Then there's the one with the caption "Jean-Paul Sartre's answering machine" and the cartoon of the machine saying, "I'm not here. You're not there. There is no beep. Don't leave a message."