I agree with Kathryn that we should try for a pro-reason slant rather than an anti-religion slant. Atheism is too often thought of as a purely negative position. I also like the "Imagine Reason" slogan.
And, for a laugh, here are my Dad's suggestions:
"If you believe in a god, you're an idiot."
"Grow up, you morons."
"There is no God. Deal with it."
"The great religions of the world can't all be right. But they can all