Coming out as an atheist to your co-workers might sound a little scary, especially if they are hard-core believers. First of all, don't feel you are obligated to share your atheistic view point with anyone if you don't feel comfortable doing so. It is a personal matter and should only be shared when and how you feel comfortable.
However if you do want to come out as an atheist try doing so with someone you already have a good rapport with. Dropping the A-bomb at a company function might not be the best first outing for a closet-atheist. Prepare ahead of time a simple explanation. The one I often use with a known believer is: "I'm not yet convinced of the existence of god, but as soon as they discover some evidence, I'd like to be the first to know." This is a kind of "soft" outing and I have never had a harsh reaction from it.
You'd be surprised how many people have never met an "open" atheist before. If the person questions you, or even goes so far at to challenge you, take it as an honor. You get to be an ambassador for your convictions! Many atheist pride themselves in being rational thinkers, so this is your chance to shine. Never get defensive, emotional or argumentative. Your goal shouldn't be to change someone's mind, but rather to help someone better understand your viewpoint. There are many agnostics running around who will enjoy a philosophical discussion. Remember that a positive first interaction with an atheist will go along way for the community as a whole.
When it comes to people of deep faith, I urge you to take special care to not put them on the defensive with disparaging comments or throwaway quotes about: "religion being the opiate of the people." That is the surest way to make someone climb on their pulpit and get defensive very fast.
In my experience, it's best to have a "to each his own" attitude. After all, we are talking about the ultimate unknowable here. While we atheists have pretty much made up our minds that there is no god, other people have made up their minds that there is a god, and that is their right too. If you ever find yourself backed into the debate corner by a believer, I find it's useful to remind them that the question of god—so far as we know—is still a mystery.
You will never know the result of coming out as an atheist to your co-workers until you come out. There is always a chance that this news will be met with an indifferent shrug and have no impact at all. Maybe it will be a positive, self-affirming experience. (I do hope so.) What it should NEVER be is an excuse for your co-workers or your boss to discriminate or harass you. Unfortunately atheists are still a somewhat unpopular minority in some areas. Luckily we have freedom of religion laws in this country and so any unlawful treatment should be reported to the proper authorities immediately.