I have always liked this image and I think it would act perfectly as an atheist symbol - in a smaller format, perhaps, just the trash can with other religious symbols contained within would suffice.
Those of you who said you were atheist and didn't need a symbol, what about this one? I don't particularly need it, but it symbolizes what I really feel - that we're better off not clouding our minds with delusion of any sort, religious or not.