allan: Actually for a bunch of atheists, they don't sound very sure about it...
You are mistaking atheists for those who have the negative belief (in theistic areas), rather than those who hold no belief (in theistic areas), which is the definiton of atheism.
Granted, it's a difficult and sometimes grey distinction depending on who you talk to, but the statement on the billboard is totally fine with atheism.
There probably is no God (there probably is no FSM, Main-in-the-moon, or marsh mellow in the center of the earth), so stop worrying about it (because it's pointless to believe in something for which no evidence exists and which is neither provable or disprovable).
Tenuously still on topic, this thread is about religion right.