Here is a recent book from Richard Dawkins, a scientist who has become kind of a poster boy for atheism. Pretty strong title for someone who is not making a positive assertion that god does not exist:
Amazon.com: The God Delusion: Richard Dawkins: Books
I'm not making the assertion that no atheist has ever asserted that god does not exist without a doubt. I'm sure some have.
In regards to Mr. Dawkins book that you referenced, he actually outlines in the book the continuum of belief in god on a scale of 1 to 7, 1 being completely a faithful believer and 7 being a complete atheist (or I may have the 7 and 1 reversed). Mr. Dawkins says in this book that he is technically an agnostic (something like a 6.99), since one can never truly conclusively know for 100% certain that god does not exist. He allows the possibility, with strong enough evidence, that god exists. Basically the same beliefs that I have. With incredible claims should come irrefutable evidence.
The God Delusion was actually a great, well written book. Very interesting if you are into theology, religion, or apologetics (and on either side of the debate). This particular book was a good, easy, quick read, unlike some of Mr. Dawkins' other works focused more on evolutionary biology that were pretty heavy on the science and info (yet interesting and rewarding once read). It comes highly recommended regardless of religious affiliation. But you probably won't agree with everything he says (I don't either).
If someone told me they were an agnostic, I would assume that their mind is not made up whether they believe in god. They are unsure. They have reviewed the evidence in support of the existence of god and cannot come to a conclusion as to whether to believe or disbelieve. Contrast that with the atheist that does not express a belief in god. I'm sure many claim agnosticism out of convenience. It doesn't sound as harsh as "atheist". They can always say to the evangelist "well, you might be right".
From my point of view, god either exists or he doesn't exist. My (or your or anyone's) subjective belief in god doesn't alter the reality of his existence or nonexistence.
And arguing about the existence or nonexistence of god does not address the question of whether good things can come from religion.
Back to your statement from an earlier post: "[Atheists] positively assert that there is no god, not that there is no proof that there is a god."
I think the response there is that the atheist sees no proof in favor of the hypothesis that there is a god (clearly great minds have disagreed over this point). And in the absence of any evidence to believe that there is a god, one must logically reject the hypothesis that there is a god based on the evidence. The rejection of the hypothesis of god could always be reexamined in light of new evidence, which is why I think all intelligent atheists are technically agnostic, if ever so slightly.