I'm not a fan of "slap in the face" atheism. That is to say, I prefer to be much more subtle about it. Instead of getting specifically atheist tattoos, I've been getting "science" tattoos, since it was my thirst for knowledge and love of science/evolution that led me to atheism in the first place.
So far I have a large atom on my right calf. I plan on getting Darwin's "I think" tree (but haven't decided where yet), the "ascent of man," and a double-helix band around my arm.
If I do get something specifically atheist, it will be this quote by Douglas Adams: "Isn’t it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?"
I do also like the IPU symbol, and have toyed with the idea of getting that done somewhere, outlined in black and filled with pink, fading gradually.