The American Religious Identification Survey 2008 results are online.
One interesting note. In terms of “belonging,” or self-identification, 0.9% of Americans describe themselves as agnostic, 0.7% as atheist. But when asked about actual beliefs, 2.3% of Americans take an atheist position, while the “hard” and “soft” varieties of agnosticism add up to 10.0%.
Interestingly, there is an additional 12.1% who have a deist, “there is a higher power but no personal God” view. There has been slight growth in the percentage of nonbelievers and unaffiliated since 2001, but nothing significant. It appears that the unaffiliated group had a burst of growth in the 1990′s, but this has pretty much stopped. Make of all this what you will.