I just want to clarify something. I find the reaction that some people have when I say (as I fairly often do) that I'm an atheist... well, disappointing. The most common responses, particularly from those who are theists, are "well that's just another religion" or "you're just a another kind of fundametalist". These are reactionary, unflatteringly glib, and smack of indoctrinated aversion. So I'd like to take this opportunity to clarify a few things.
Atheism is not a religion. It is the conscious, deliberate absence of religion.
Atheism is a religion like water is a cocktail, or "off" is a TV channel, or baldness is a hairstyle (yes, that last one hits close to home :)).
Theism is the belief or faith in a supernatural deity or god. Pantheism is the belief in many gods.
The "a" in atheism is the greek prefix for negation. Just like "asexual" means having no discernible sexuality, and amoral is "without morals", a-theism is "without belief in a god or gods."
Atheism does not mean "believing there is no god". That is a dumb statement. It is impossible to prove non-existence. To state that "there is no god" is at least as presumptuous (and indefensible) as it is to state "I believe in the one true God" (given the total lack of evidence to support that claim). Those who catagorically state that "There is no god" are anti-theists or non-theists. I do not align myself with them.
Stating that one does not believe in a god is a completely justifiable statement and signals a sensible scepticism which stands up to all scrutiny. It is also certainly defensible to suggest that the likelihood of there being any particular God (as in a Christian or Judaic God or an Islamic Allah) is no greater than there being a Santa Claus or a toothfairy or - the canonical staples of the sceptic - a celestial teapot or a Flying Spaghetti Monster (personally, the former is a bit staid and the latter sounds unnecessarily messy). I prefer to think about the disciplined investigation and inspiration which characterise science (when practiced ardently and with intellectual honesty).
Agnostics are those who do not know whether there is a god or not (which should be all of us). Being an agnostic is not incompatible with being an atheist, but agnosticism tends to lend an undeserved credibility to theism by implying that it's about an "even chance" of there being a god to believe in or not. Personally, I lean far more strongly to the "vanishingly small to the point of insignificance" likelihood of the existence of any of the gods of our world's major theistic religions with their legions of credulous followers.
For those who would use the argument that "if so many people believe it, there must be some truth to it" argument to justify their own theistic faith, I simply suggest you consider the time, not too long ago in the grand scheme of things, that people en masse knew that the earth was flat, or that there were four elements (earth, wind, fire, water), or that the devil (or at least evil spirits) would occupy your body if you didn't receive a blessing after sneezing (Viruses? Who could possibly believe in something so absurd?). Thankfully, in each of these issues, scientific theories have been completely accepted by almost all of the world's cultures.
As I like to say to believers, particularly those who drop in unannounced at my house on a Saturday morning (I sometimes allow myself the hint of a smirk): "Your God made me an atheist. Who are we to question his plan?" For those evangelists with the guts to actually consider the statement, its logical implications tangle them up in the funniest possible ways.
Another thing about which I'd like to be clear: atheists do have beliefs (in things which cannot be usefully proven, e.g. market economics, and the kindness of people), and we enjoy the wonder of life around us and contemplating the universe. We just don't feel the need to ascribe any of it to the hand or grace of a god or gods. It can just be appreciated at face value, and as something which can, in theory at least, be understood eventually, by someone.
Also, atheists aren't against morality nor are we amoral. We tend to bristle when people suggest that the arbiter of moral teaching in our society is one or another church. This, I'm afraid, is simply false. I would suggest that the moral teachings in the churches of the world are not what give society its morality - they instead reflect the mores of our society. So do our schools and every other social institution. Some people value goodness and accept the burden and joy of adopting a moral backbone, others do not. Whether they attend church or believe in a god is, by and large, completely irrelevant. Atheists are among the most principled and morally conscious people I know. The fact that some people ascribe their morality completely to their faith in God (and presumably the prospect posthumous enticement/threat of heaven/hell) is a sad indictment on their innate moral fibre. People should be moral because it's "the right thing to do" without any external or supernatural incentives.
I can't resist including this thoughtful statement by Richard Dawkins (who, love him or hate him, is an admirably clear thinker and sceptic with a gift for the soundbite) in The Root of all Evil: "We are all atheists about most of the gods that societies have ever believed in. Some of us just go one god further."
Right, now I'm about to board (fingers crossed - it's already been delayed twice) a plane bound for the island continent. I don't believe in "a wing and a prayer". I can fly thanks entirely to science, and that's God's own truth.
Follow up (2012-06-30): yep, left shortly thereafter, and despite my blasphemy above, survived the return flight as well.