What I might say to:

I confess I don't have the stamina to wade through all eighty of the previous posts, so if somebody has already made this obvious point, I apologize. The OP is assuming that an atheist is asserting that God doesn't exist, and therefore should have proof of that assertion, since he who asserts must prove. That would be true of "hard atheism" which is an increasingly rare position these days for the obvious reason that it's impossible to disprove gods, ghosts, unicorns, Spaghetti Monsters, etc. Jean-Paul Sartre is a good example of hard atheism when he says flatly "Le bon dieu non existe pas". We could legitimately say to Sartre "that's a pretty bold claim. Can you back it up?" Sartre's phenomenological perspective is far removed from the empirical tradition of modern naturalistic atheism. The "Four Horsemen" of New Atheism (Dawkins, Dennett, Hitchens, and Harris) verge on hard atheism, but even they acknowledge that the non-existence of God is a probablility instead of a certainty. "Soft atheists" simply say they don't believe in God--i.e., no evidence convinces them that God exists, and our current understanding of the way the universe works persuades them that it is. For example, theoretical physicist Stephen Hawking's new book, Grand Design, argues that the concept of God is unnecessary for a scientific understanding of the universe. Elsewhere he acknowledges that there could be a God if God is identified with the laws of the universe, but he doesn't choose to go that route.

How does this differ from agnosticism? I'd say only in degree, and in the way a person chooses to live his/her life. If a person acknowledges the possibility that there might be gods or ghosts, but lives as though there aren't any, that, to me, is the sign (s)he is an atheist and is willing to bet his or her life on it. If a person lives as though either possibility is equally plausible and tries to suspend judgment, I'd call that person a true agnostic. If a person (like me) thinks there's enough evidence to bet on the existence of Something Big Out There, and to live accordingly, I'd say we have a believer on our hands, whether we're talking an involved Theist God, a laid back uninvolved Deist one, or some Pantheistic concept like the Gaia Hypothesis.