Atheism is the logical conclusion of Agnosticism. If you call yourself an Agnostic but not an Atheist, you are actually an Atheist. Unless you use a different definition of the words than I do, so let’s get our definitions straight.
There is a common misconception that an Agnostic is someone who cannot decide what to believe - or they simply don’t believe or disbelieve in any religion; these are wrong. An Agnostic is really someone who believes that God(s) cannot be proven to exist, nor can God(s) be disproven to exist, given our present knowledge. A basic argument an agnostic would use is that you cannot deduce that God(s) exists from logic alone. eg the ontological, cosmological, transcendental, teleological, argument from morals, argument from miracles, argument from scripture, argument from personal experience are all flawed (elaborations on these flaws abound, just google them). And as long as God(s) doesn’t come down and introduce himself there is no way to prove it exists.
But in addition there’s no way to prove that nearly any out of infinite possibilities exist or don’t exist. For example, you can’t prove there is or isn’t a Flying Spaghetti Monster, a Zeus, a Flying Purple People Eater, or a Bigfoot. Therefore an Agnostic is not on the fence about God(s) - an Agnostic KNOWS that there is currently no way to prove or disprove God(s).
Given that there is no evidence for the Flying Spaghetti Monster, Zeus, a Flying Purple People Eater, Bigfoot, or God(s) the rational conclusion is that until sufficient evidence exists, they do not exist. The idea that the burden of proof is on the believer bridges the gap from Agnosticism to Atheism.
Unlike propaganda claims, an Atheist is not someone who hates God(s) and is immoral. Atheism literally means “without Theism”. That is all. An Atheist is someone who simply does not believe in God(s). It doesn’t say anything about what an Atheist is (in an affirmative sense). “Atheist” only describes what someone isn’t. Some people call this “weak atheism”. It does not automatically mean that an Atheist disbelieves in God(s) in a definite sense, aka “strong atheism” (although she might).
If an Agnostic is a logical person, she knows that you cannot prove or disprove a God(s) just like unicorns and Zeus. The logical conclusion is disbelief until evidence presents itself. Therefore, an Agnostic is without Theism and is an Atheist. But given the social stigma, they may lack the courage to use the label to describe themself.
Updated: 2015-09-26. The main ideas are unchanged, I’ve just added detail and clarity.