Yes, I've actually come across that one a few times. Perhaps this surprises you because you, like me, have lived and worked in the South and didn't hide your atheism at all and didn't even realize how brave you were being.
But I can understand the atheist closet. Sure I can. After all, Rosa Parks said, "I sure am tired after cleaning wife folks' houses all day long, and there are some empty seats up there in the front of the bus -- but since I live and work in the South, I'd better keep standing up here in the back and not rock the boat."
No, wait -- that's not what she said at all, is it? And Martin Luther King Jr didn't say -- to himself -- "I have a dream -- but because I live and work in the South, it would be better if I didn't raise a fuss about it." And the guys in the B-52's didn't decide to tone down the gay, to be on the safe side, because they lived and worked in the South. And if you haven't heard about it already, check out the paint job the Westboro Baptist Church's newest neighbors gave to their house. (God hates fags? Well, people hate bigotry. And people are real.)
Now, I know, some of you may be saying, "Hey, Steve, your comparisons are skewed, because ethnic minorities and gays are at much more risk for all kinds of discrimination than atheists -- even here in the South, where I live and work and loudly make fun of believers on a daily basis!" And of course you'd be right to say so, which makes these closeted atheists even bigger pussies, which is kind of my point.
Whether they suffer from racism, sexism, homophobia, religious bigotry or some other form of insanity, tyrants have only as much power as is given to them, by cowards.