There's been some recent disagreement about the desirability or even the meaning of "spirituality" from within a naturalist framework. The word itself seems to imply a supernaturalist dualism of body and spirit. A naturalist, of course, must reject such dualism.
As we see from Chris Mooney's article in USA Today, though, some of the most ardent naturalists have been willing to embrace the term. The question then becomes what the term "spiritual" means in a non-dualistic, naturalist framework. Richard Dawkins describes it as: "a sort of sense of wonder at the beauty of the universe, the complexity of life, the magnitude of space, the magnitude of geological time. All those things create a sort of frisson in the breast, which you could call spirituality." Dan Dennett says he feels spiritual when he's "just transported with awe and joy and a sense of peace and wonder at, whether it's music or art or just a child playing or some other wonderful thing off of my sailboat, being amazed at the beauty of the ocean."
So it's something like a sense of awe and wonder. I'd like to be a little more precise, though. It seems to me that in the relevant sense, a "spiritual" feeling is one that reveals a certain sort of relation between ourselves and the universe. It's a frisson or feeling that accompanies our awareness of our smallness, and in particular the smallness of our daily worries and preoccupations before the immensity of reality. It is our awareness of the insignificance of all human worries and preoccupations; the sense that we are all but a mote of dust in something vaster than we are capable of completely grasping.
That is a feeling that a naturalist can share with a non-naturalist. Indeed, I'd argue that naturalism is actually the way to approach this sort of spiritual feeling par excellence, since it is only through a scientific framework that we begin to grasp the true vastness and grandeur of reality, rather than finding phantoms of it through images of human fantasy.
But there are ways that human fantasy does approach a correct understanding of this sort of vastness. It can be done in literature, the arts, but one classically naturalist, or at least scientifically oriented way that it can be done is through SF. The great SF stories are intended to be spiritual in the sense outlined above. They give a sense of the sweep of time and space that is not normally found in other forms of literature, although of course they borrow tropes from all parts, particularly from forms of epic myth. In this way they meld the mythopoetic human impulse to narrative to a naturalist, hence to an extent real and accurate, picture of the universe.
I don't mean that SF is always a true and accurate picture of reality, of course. Even hard SF fudges at the corners. But it seems to me no other form of contemporary literature is more interested in notions of the spiritual in a rough-and-ready naturalist sense than is SF.