There are so many great ideas in here, I can't post in them all. I like the simplicity of this one very much, although I'd like to see a bit more in there. Maybe a famous atheist or humanist quote, followed by "We ThinkAtheist.com"?
Specific inspiration would be the UK's Atheist Bus campaign, which carried slogans such as "There's probably no God. Now stop worrying and get on with your life" on the side of buses.
Some suggestions: "Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric." - Bertrand Russell
“Atheist: A person who believes in one less god than you do” - Donald Morgan or, similarly:
"I contend that we are both atheists. I just believe in one fewer god than you do. " - Stephen Roberts
"It will not do to investigate the subject of religion too closely, as it is apt to lead to Infidelity." - Abraham Lincoln
Personally, I like the first. The others work but could be seen as being a little more confrontational, and I think it would be nice to have the higher ground when The Great Offended arrive. I like the Lincoln quote but (however illogical) it may well just lead to cries of "Un-American!", which phrase has never been known for contributing to a useful discussion.
What do you think? Any ideas, or quote suggestions?