It does not matter whether one is a non-believer, because one has no concept of God or because of a consious decision - both cases are Atheism.
If the baby never learns how to drive a car and thus never makes the driver's license she will never drive a car for the rest of her life - whether with 1 year, 15 years, or 21 years.
If the baby is not indoctrinated or actively influenced, chances are high that she'll stay a non-believer. And the older she gets the more likely she won't adopt a belief. Such a scenario is highly unlikely in the US, where you cannot escape religion, but if you take certain Scandinavian countries, not believing in a god is more or less the norm.
Here in Germany I know lots of people who were raised a-religious (careful, I don't mean anti-religious!). Most of them stayed non-believers for the rest of their lives - just because their parents did not care about whether they believe or not.
So the default state is IMHO that which is the case when we come to this earth and requires something to happen in order to change.
Babies might not be atheists according to your definition of atheism. But that's the "problem" - people define things differently.
I don't presuppose a concious decision for being an atheist (although in lots of cases there is a conscious decision). An Atheist in my book (and according to the majority of most Atheists I know) is simply someone who does not believe in a personal god - which is not dependent on the fact that it was a conscious decision (e.g. a Christian who does not believe anymore) or not (someone who grew up in a neutral environment and never developed a belief).
We would also have to define what we mean by "god". This requires characteristics/attributes, otherwise the term "god" would not make sense.
If you take Brad Warner's definition - then yeah, you can call me a believer!
If you talk about a god who had a son of flesh and blood that came to our planet, born by a woman that was still a virgin at his birth, then call me an Atheist.
The same holds true for Thor, Zeus, Osiris, etc.
Anyway, we are running in circles.
As Hans suggested we should split this thread.
Before I bow out of the discussion, I'd like to recommend these two short posts:
http://www.atheistrev.com/2009/04/what-is-atheism.html
and
http://www.atheistrev.com/2008/03/al...-atheists.html
Just a short disclaimer: I don't know any of the other posts on that page, so what you'll find there won't necessarily be shared by me. I found those two posts helpful though.
@Hans:
Sorry for that discussion - I will read Harris' book, thanks for the recommendation.
Gassho,
Daitetsu