I wear a bunch of nerdy shirts (from xkcd, webcomics, thinkgeek) when I'm not at work (where I have to be business casual) and I'm really pleased by the effect! I've gotten into conversations with people who recognized the source, discounts on coffee from the nerdy barista, and I try to hold up my end of the bargain by saying "Nice shirt! I love [X]" to other people wearing similar shirts.