Sponge-headed Scienceman reports on a Rice University study revealing that 17% of atheists attend church at least occasionally, most of them "for the kids." He comments on why this might be. I concur with his reasoning, but would add that this group may indeed be one which is open to different ideas, as they claim about themselves. Rather than being "believers" in some unexamined sense, who declare they believe in God but somehow never get around to doing anything about that, this group might have decided that on balance they cannot believe, but allow the possibility there might be something to it, and are the sort who are quite intentional in tying their behavior to their beliefs. Perhaps I am painting them too rosily, imagining their thought to suit my own beliefs.
I am reminded of Michael Novak's essay of four years ago about different types of atheists - he identifies six. These are not entirely distinct, but overlap, and in some cases overlap with believers. I have known people in each of the first five categories - no one comes to mind for the sixth. I suspect one would have to be a very thoughtful person, aware not only of the beliefs around one but one's own character as well, to make it through to this conclusion.