I'm immediately interested, then, in any attempt to create a symbol, if not ex nihilo, then the next closest thing. As an example, consider Norwegianity's call for a logo for freethinkers [scroll down], which has been echoed and amplified by PZ Myers.
Inventing a symbol is hard. It has to be unique but not unrecognizable, bold but not garish, simple but not obscure. PZ explains,
Anyway, the kind of thing I would be looking for is something simple, fairly abstract, easy to render, and that wouldn't antagonize deists, agnostics, or atheists. It should be positive: no crucifixes with a slash through them, for instance. It shouldn't be weird—no flying spaghetti monsters, please—it shouldn't be ugly, it shouldn't be in-your-face and gloating, it should be unobtrusive. It ought to be the kind of symbol that if it were done up as a piece of jewelry, it would be tasteful. Remember, even if you do come up with a nice logo, the hard part is going to be getting a critical mass of unbelievers to adopt it and build a recognizable association with it (and be warned, no matter how gorgeous and elegant and clever an idea you come up with, there will be a solid cadre of the godless who will resolutely refuse to have anything to do with it, on general principles and intrinsic cussedness…which is OK.)