Atheism isn't very widely accepted or thought of very well by large numbers of people, especially in the United States. As discussed yesterday, even some liberal areas can be filled with hatred and hostility towards atheists. Because of this, many atheists feel compelled to keep their atheism a secret, not telling anyone they don't trust. Even family members might be kept in the dark. Do you hide your atheism, or are you 'out of the closet' and open about things?
It would probably be better for more atheists to be more "out" and open about who they are and what they believe. The mere act of keeping it quiet helps reinforce the idea that atheism is something to be ashamed of and therefore is "wrong." In addition, the more contact people have with atheists, the easier it will be in the long run to dispel popular myths and misconceptions about what atheists believe, what atheism is, and so forth.
On the other hand, when people perceive legitimate threats to their livelihood, grades, or even health from god-fearing Christians in their community, it's difficult to tell them to come out of the closet anyway. For some atheists, being out and open just isn't a very realistic option. For that reason, I think that atheists who don't face any strong threats have a greater obligation to come out and those of us who are already out have a strong obligation to be more open about our atheism -- we need to make up for those who can't and make it easier for them to eventually come out.