A fascinating debate has gone on on a rec'd blog about definitions of Atheism and Agnosticism, and "belief systems". Perhaps a bit of a more tangential perspective might help, to understand that there are ways of thinking about the real world, and imaginary worlds at the same time, that aren't value-saturated "belief systems".
I never refer to myself as an "Atheist". It's not a question to me whether any type of god exists or not. While I love the idea of the supernatural - is anything more fun really than perhaps meeting Pixies, your Great Great Grandmother's Ghost, Santa Claus, Pan, and perhaps a Sylph or Incubus tossed in - I know supernatural beings are of course all fictions. There are other types of supernatural concepts - curses, hexes, magic, blessings and miracles; souls, reincarnations, possessions, demons, angels; esp, telekinesis, seeing the future; good/bad luck - breaking mirrors, 13, walking under ladders, spilling salt, stepping on a crack, black cats; and then finally aliens, yeti, loch ness monsters and the ilk. And then on an entirely different plane - astrology, tarot, tea leaves, ouija, and perhaps modern ideas - the videotape that can kill upon viewing, the website that does the same...
All fun, and wonderfully silly, and of course purely imaginary phenomena. So, there are those of us who don't have a systematized "lack of belief" in a God. It's more of the lack of credence to the entire category of supernatural. The Christian God exists surely as the bad luck of the number 13 in the minds of believers in the supernatural. So the concept of a god, or bad luck 13 to us is a human system of supernatural thinking that as a whole is fantasy - along with Flying Spaghetti Monster. So whenever you get in an elevator and note that the number 13 is missing, or see in a magazine or newspaper an "astrological chart", note a book on "Feng Shui" in Barnes and Noble, hear a "bless you" after a sneeze, see a kosher or halal food sign, or hear a bell tolling on Sunday morning at a church, realize that you're seeing a manifestation of different varieties of the same thing: the boundless desire for humans to create and interact with supernatural ideas as though they were fact.
In many ways Atheism exists (I think of the Hitches variety) in dialectical opposition to religion. I think the synthesis lies somewhere in thinking about both as views on a natural world.
If the word weren't already used, I think the better term for my thinking (better than Atheism) would be 'naturalist' as opposed to 'supernaturalist'.