Recently, I've been forced to defend my atheism. That doesn't come as a big shocker to me, given that I live in the 'south' but what does surprise me is why I've been forced to defend it. It all started when a pack of youth missionaries asked me "How can you be so certain that there is no God?". I told them I wasn't. For all I know, there could be a God but I simply don't believe there is one. Like sharks smelling blood, they jumped all over that one for my apparent self-contradiction.
Such ignorance (and I don't mean that in a derogatory fashion) isn't relegated to the square walls of the coffee shop I frequent, nor does it fall within the confines of the Mason-Dixon line. Indeed, I've read several blogs and forum posts that seem to have a point confused. That point is, namely, that atheism and agnosticism are mutually exclusive. In other words, many seem to think that either you are an atheist or an agnostic - you can't be both. It would be like saying you're a married bachelor or a compassionate conservative (just kidding...sort of).
So let's get something straight: agnosticism and atheism are not mutually exclusive. One can be an agnostic atheist (hey, if I can do it, you can too). My reason for saying that is fairly straightforward - agnosticism and atheism make completely different claims regarding completely different levels of cognition.
Agnosticism (from the Greek - a (without) gnosis (knowledge)) is a claim concerning itself with knowledge, or more put more aptly, the lack of knowledge. One can be agnostic about a great number of things: the number of jelly beans in the jar, the number of cars on the road at a given time, or the true rationale for war with Iraq. In these circles, however, agnosticism usually refers to the existence of God. Someone who claims that they are agnostic when it comes to God is simply stating that they don't know. Could be. Could not be. More specifically, many agnostics (myself included) hold that it is actually impossible to know whether or not God exists.
While agnosticism makes a claim regarding knowledge, atheism makes a claim regarding belief - namely, I don't believe God exists (or you can put it into the affirmative if you prefer, e.g. I believe God doesn't exist). However you want to sexy it up, you're basically saying the same thing: we're godless creatures in a godless universe.
The key difference between these two notions is the difference between knowledge and belief. Belief is a sort of substitute for knowledge with respect to things that are not yet known or are inherently unknowable. In other words, one can believe something without having knowledge. For example, I can believe that there are one-hundred fifty four jelly beans in the jar, or that there are two point six billion cars on the road, or that the real rationale for war with Iraq was to feed the military-industrial complex.
The things one can know, however, is a bit trickier. If you really examine what knowing something means, you'll come to the conclusion that the scope of things we can actually know is somewhat limited when compared with the popular use of the word 'knowledge'. Perception can be in error. Memory can falter (see Elizabeth Loftus for some interesting work on implanting false memories). We can be mistaken in our reasoning. But, of things we can know, it would seem odd to say that one believes them. For example, I know that two plus two equals four (I got an 'A' in pre-algebra). It would seem odd for me to say that I believe that is the case. Or, an old argument that St. Augustine used to trash the Skeptics - I know that I do not want to be in error. It would likewise be odd to say that I believe I do not want to be in error.
The point that I'm trying to get at here is this: belief and knowledge are different operations that are concerned with different levels of epistemic certainty. If you don't believe me...well, I guess I can't make you.
You speak of epistomology, our systems of knowledge. But don't all systems of knowledge come down to belief? Doesn't a strict rationalist hold the belief that the fundamental axioms of science are inherently true? They can't be proven, after all.
So given this question of atheist versus agnostic, I think of this as shorthand:
atheist: my system of knowing things is based on scientific principles, however as there are things which can't be explained through current science, it is my belief that they will eventually be understood through science, not by god.
agnostic: my system of knowing things is based on scientific principles, however as there are things which can't be explained through current science, I accept that one possible explanation is god.
So, in this sense I see these two points of view as mutually exclusive.
Don't we accord beliefs acquired through experience and experimentation with greater value than we do simple belief?
And I would say if a god were to exist, that by the very criteria we seem to have for gods then any god would outside of the universe and therefore could be no naturalistic explanation. Such a god would be unknowable -- either way. I'd posit that all honest people are simply agnostics in relation to everything, just with varying confidence levels.
Maybe it's because it's late here and I'm just not reading you correctly...but huh? I think you have a point about the things we actually can know...perhaps we can actually know very little. But I'm having a hard time grasping how atheism is a 'system of knowing things'. My basic point was that it's a system of belief. It's not a mindset concerned with explaning phenomena, but just an assertion that there is no God. Taking steps toward scientific investigation requires that further claims are made, IMHO.
Here's a really simple example I use to explain myself on this issue--
I do not say that I am agnostic about the existence of the Easter Bunny. I say that I do not believe the Easter Bunny exists. If I say that I am agnostic re: the existiing of a particular or a generic deity, then I am ascribing more plausibility to that belief than I am to the Easter Bunny. Such deference to one belief is unwarranted.
So if you want to get technical about things, yes, I'm "agnostic." But for all intents and purposes I am atheist. Now that you mention it, I actually prefer "anti-theist," since it's my position that theism is demonstrably harmful.
One thing that really bugs me is when people act as if the statement "I am an atheist" means that you have declared yourself absolutely intransigent and incapable of responding to argument...and they tell you that being an agnostic means they are less close-minded. It's not the case. Being an atheist means one has made an unambiguous statement of one's position -- it does not mean that that position is refractory to evidence or is anything more than provisional (as any scientist knows, all knowledge is provisional).
I'm an atheist. That means I have reached a reasonable conclusion from available information (or lack of evidence for theism), and am sufficiently satisfied with the rationale for atheism that I will commit to that position. It's not about metaphysical certainty at all. It's about taking a stand in an argument -- and if people want to budge me from it, all they have to do is provide evidence for an alternative. I'll also add that by taking a solidly polarized position, I should be making it easier for critics to counter my position. Isn't it revealing that they can't?

One thing that unites many athiests and agnostics is a naturalistic worldview. The term 'bright' has been recently coined: a 'bright' is a person who has a naturalistic worldview. A bright's worldview is free of supernatural and mystical elements. An antonym that has been suggested is 'super' - for those whose worldview contains supernatural elements. More info can be found at:
http://the-brights.net/