Philly.com ran a poll next to a Philadelphia Inquirer story about a PhillyCOR billboard on I-95. The question: "Do you believe in God?"
YES 4039 (27.7%)
NO 9585 (65.7%)
Not sure 967 (6.6%)
As much as I would love to believe this, the likely response is either (a) one person voted many times, or (b) only people predisposed toward not believing in God read the article, then took the extra step to vote in the poll.
My barely educated guess, if everyone in the US voted, would probably be:
NOT SURE 20%