I'm going to change the topic a little. A number of pages ago, someone asked if you can be atheist and spiritual. I would say yes, certainly, though I don't think that I'm spiritual as such.
I think enwiabe's points about uncertainty are very good, but you also have to keep in mind that many atheists are certain about how the world works. It started with the big bang, it's comprised of atoms and energy and such. When you die there is nothing. While I disagree with the way religion gives certainty to the universe, I also disagree that we can speculate what happens after we die. And further, while I definitely believes in the findings of science, I don't think it gives us a full illustration of the world.
Although I don't identify myself as a spiritual person, I am interested in the intangible. In ideas, in metaphysics, I'm interested in things that aren't directly observable. God could be one of those things, but I personally believe that the idea of God is much more important then his or her literal existence. What character, (in one form or another or many) boasts quite so much influence over human civilisation? We can see God everywhere in churches and synagogues and mosques, but God gets to the heart of our laws, our beliefs, our idea of what's right and wrong. God is in the Declaration of Human Rights as much as he is in the church. Even those who don't believe in God are so deeply influenced by the idea of God, because this idea has so much to do with out culture. Putting on my anthropologist hat here, many of our ideas, particularly pertaining to morality, are based on Judeo-Christian sentiment. God is very real (maybe not literally though).
But anyway, I think that my interest in the humanities, in philosophy, in art, etc. means that I don't see the world in a reductionist kind of way. There's more to the world than its physical/biological/chemical nature. There's beauty and power and emotions and lots of over abstract stuff. For me, this abstract level of intellectual inquiry replaces spirituality for me. That's why I actually identify myself as a Secular Humanist before I see myself as an atheist. I think that there is more to the world than what we can observe, but I don't think that necessarily means that there is a literal God out there who created us and watches over us.