So atheists make up about 0.21% of the prison population?  Interesting.  That's definitely much lower than the percentage of atheists in the general US population.  Admittedly, demographics of atheists vary widely depending on the source.  The CIA World Factbook puts the figure at around 4%, while a 2004 BBC poll puts it at around 9%.  A Center for Inquiry podcast a few months back cited a poll putting those who explicitly called themselves atheists or agnostics at just over 10%, though I'm unable to find the source for that (and don't care to listen to several months' worth of podcasts to find it again).

I've also noticed that figures are open to interpretation; some sources explicitly distinguish between atheist, agnostic, and generally nonreligious/secular, while others lump them together.  This Pew poll puts atheists/agnostics at 1.6%/2.4% respectively, but also finds 12.1% "nothing in particular."  A 2001 ARIS poll puts atheists/agnostics at 0.4%/0.5% but an additional 13.2% for "nonreligious/secular."

Also check out this link for tons more statistics concerning atheists.