I found these graphs on Sam Harris' website, from an online survey he conducted in May. Since the survey didn't use random sampling, these numbers shouldn't be treated as accurate, but it's still pretty interesting. There were about 5000 respondents, mostly Atheists, of whom about 250-700 were Christians.
Could be that Atheists are generally of more-than-average intelligence...or that they're more full of themselves...
Makes sense. If you're more skeptical of UFOs, you're probably more skeptical of God. And vice versa.
More graphs here. (Scroll to the links at the bottom of the page.)