In Breaking the Spell, Daniel Dennett claims that atheists are as law-abiding as believers.
The MIDUS study asked people their religious affiliation and if they had ever been in jail. The results (sample size = 2,183):
Percent ever in jail
No preference 13.0*
Has a religion 4.8
*significantly higher than people with a religion
Zero of 18 atheists report any jail time. By contrast, agnostics, spiritual people, and those with no religion have significantly higher rates of incarceration. On the various measures of functioning that I've studied, the squishy middle categories often have the worst numbers.
UPDATE: Keep in mind that the current trend is toward more people having no religious preference (but still believing in God) or saying they are spiritual rather than having a religion.
While we're at it, MIDUS gives us another estimate of the percent of Americans who are atheist. The survey asked people to describe their religious affiliation, and "atheist" is one option. Only .8 percent of participants gave that answer. (The study was conducted 2004-2006.)