Just want to add something to this.
Palpitus wrote:
Theism: God exists
Atheism: God does not exist
Agnosticism: God may or may not exist (depending on definition of agnosticism)
I don't really agree with your definition of agnosticism (yeah, I'm aware you put a conditional on it. I'm going to explain the other condition). I also think that's largely why many people get confused over the whole athiest/agnostic positions and tend to blend them.
Theism, as you stated, is an absolute believe in some specific divinity. "I believe in God". "I believe in Thor". "I believe in the Great Pumpkin". All are exampes of Theism. Remember though that theism is a "personal" belief in a very specific supernatural thing. A vague belief or acceptance that there may be some divine or supernatural force that we can't explain is *not* theism. It's actually closer to agnosticism (depening on *why* you believe something).
Atheism is the exact opposite. It is the specific belief that any given divine/supernatural force does not exist. An Athiest disbelieves *all* forms of Theism. So he doesn't just not believe in God. He believes there are no gods, spirits, paranormal stuff. None of it. Guess what? If you don't have that absolute of a non-belief in anything, then you aren't really an athiest. You may call youself one. You may even believe that's what you are, but you aren't.
Agnosticism is simply the position that since we can't prove or disprove God or gods, or anything supernatural, that it's pointless to argue about it. An agnostic, when asked if God exists, will typically take the position that it doesn't matter. If we can't determine if God exists, and there's no measurable effect from believing or not believing in God, then what's the point? Agnosticism is most definately *not* about not being able to make up your mind. The mind is made up. There's not enough evidence to support the presupposition that God exists, so therefore, from my perspective, he doesn't.
The real difference between agnosticism and atheism (and where you're partically right) is that the atheists believe is just that: A belief. It's just as arbitrary and absolute as the theists. The agnostic, on the other hand, if you could ever show him enough proof, would accept a given supernatural thing (like God perhaps) as fact. But with a lack of proof, and given a choice of making dramatic changes in his life based on something that can't be proven, or ignoring that belief and just living his life based on what he can see and hear around him, he'll chose the latter. Sometimes his own choices may match those of the theists, but he's not doing them for the same reasons. It's a subtle distinction, but it's pretty significant.
Whenever you hear someone make an argument that a lack of proof of God means that worship of God is silly and irrelevant, that person is an agnostic, not an atheist. Whenever someone does the "odds calculation" with regards to worship, he's an agnostic (when I go through each set of possible "truths" and actions I could take with those truths and conclude that my odds are as good or better not believing in a given god then picking one and believing in it, I'm exhibiting textbook agnosticism).
Lots of people confuse those though. We run into this alot in discussions like this when someone will inevitably identify themselves as an atheist, but do the odds thing I mentioned above as their reason for not believing in God. A theist will then counter that with the fact that atheism is just as much a religioius belief as theism is. Of course, that does not disprove the agnostic position (or the argument) at all, but it becomes an associative argument. Since atheism is just as arbitrary as theism, it's no better from a rational point of view. But the arguement was really about agnosticism, so it shouldn't have been hampered by the self-titled atheism of the poster.