Atheist is kind of a strong word, isn't it? Taking that kind of a stance always struck me as rigid as any belief system. I had a professor who called himself a "secular humanist," which made a little more sense.
I think that's a misunderstanding of what Atheism means. There is nothing active about Atheism: there is no believing involved, including believing that god does not exist. There is a difference between not believing in god and believing there is no god -- it's subtle, and I've met people who aren't smart enough to distinguish that difference, but it's there.
I think this is why people have invented terms like Positive and Negative atheism. Many people would argue that if you don't specifically reject the possibility of a deity that you are not technically an atheist but an agnostic.
To me, not being able to reject the possibility outright is a simple matter of logic not of personal uncertainty. But if I were to call myself an agnostic people would assume that I give credence to the idea of a deity, which I absolutely do not.
Of course, we shouldn't really need a word for it at all, there's no actual need to reject an idea that has no empirical evidence to start with. I don't require a label to denote my lack of belief in God anymore than I need one to denote my lack of belief in anything else. If people hadn't fabricated belief systems in the first place, they wouldn't need a label to identify me as a person that rejects them. I prefer to think of myself as merely unhindered by superstition and I label myself purely as a favour to others.