So let's just deal with the archaic meaning quickly. I think this describe nihilism for the most part. 'Nough said. I think the definition in '2a' fits the bill pretty well with what most of us think. Notice that there is no reference to science or even general superstition for that matter. It is quite possible to be an atheist and believe in horoscopes, ghosts, vampires, etc., since none of those things involve a deity. The only qualifier is the disbelief in a deity. Now, the definition could be a bit stronger and say deities, but I guess the assumption is that if you don't believe in one, you aren't likely to believe in others. I think this assumption is wrong. I'm also curious about the lack of an 'a' in front of deity. I think a better definition would be 'a disbelief in the existence of any deity.' I'll have Merriam-Webster get right on that!
So, if that is what a little-'a' atheist is, what is a big-'A' atheist? I think the description as provided by Crommunist is pretty good and is summarized from a talk given by PZ Myers in Vancouver a few days ago.
We’re not Atheists by accident, or because we haven’t yet heard how awesome YahwAlladdha is, but because we reject superstition and appeals to invisible authority as a basis for building a functioning society. We believe that evidence, reason, and an abiding respect for humanity is a much higher standard to which human beings should be held than the fear of a paternal sky-genie.PZ later also explained that some atheists are "nihilists, believing that because there is no God, life is therefore meaningless." I would venture to say that it is safe to assume that most people that fall into the big-'A' Atheist description tend to consider themselves as humanists, and being atheist, secular humanists. I believe it is this humanist philosophy that drives Atheistic morality.
A misconception of Atheists (I'm pretty much done with the little-'a' atheist at this point) is that since they tend to be motivated by science, that science itself drives their morals. I think it is important to note that science is by no means a substitute for any moral authority. Science is pretty much amoral, nothing but a cold hard process and methodology to discover the truth. While there are some Atheists out there that have proposed that science may one day be able to explain morals and ethics sooner rather than later. Sam Harris gave a talk at TED about this. I think Michael Specter, in his TED talk on Science Denialism said it best about science being nothing more than a process. Outside of that process, there are serious discussions that need to be had about what science discovers and those are areas of law, ethics, morality, and even intellectual property. I do not think it is accurate to believe that science alone can drive the morality of anyone, Atheist or theist. Atheists replace religion's morality with the humanist philosophy. While the Bible itself does have some very good morals in its pages, I think it fails miserably in many others, and therefore, falls well short as a basis for a moral code of conduct.