I am an atheist, and I concur that this does not imply that I have 100% absolute belief in the non-existence of all entities which could be called a "god".
First of all, from a philosophical standpoint, I cannot claim to be 100% sure of anything. Even when I am pretty certain of something, I have to technically acknowledge the fact that I may be mistaken. I am pretty damn sure, for example, that the Sun exists. But it could just be an elaborate hoax set up by a race of super-advanced aliens (or even humans, like in The Truman Show). Or we could be living in a Matrix-like simulation of a world and in the real world there is no such thing as the Sun. Or I may just have been dreaming for the past 21 years and not known it. These are all distinct possibilities, and although I have my reasons to doubt them, I cannot be absolutely sure that my current belief about the Sun's existence is true.
Likewise, I cannot be absolutely sure that something I disbelieve in does not exist. It could very well be the case that Santa Claus does in fact exist, and that there has been some massive conspiracy to keep this information from me. There is a small possibility that there is a race of leprechauns living in a remote cave in Ireland; if the evidence is presented to me, I will accept it. However, this does not mean I won't say I believe Santa Claus and leprechauns do not exist. I do in fact believe Santa Claus and leprechauns do not exist, even though I acknowledge the infinitesimally small possibility that I am wrong about that.
Same goes for God. I think the case against the Islamo-Judaeo-Christian God is stronger than the case against Santa Claus and leprechauns, and here's why. We can logically disprove some entities, because they contradict basic laws of logic or physics. For example, we can prove there is no such thing as a square circle, or a perpetual motion machine, because we know there is no logical way these things could possibly exist. They defy the fundamental laws of our universe upon which all of existence is based. The traditional Islamo-Judaeo-Christian God is logically impossible in this way. It is impossible, for example, for an entity to be both omniscient and omnipotent; if it knows all its future decisions, it lacks the power to change its mind. Therefore, we can conclude that no omniscient, omnipotent entities exist in the universe, just as we can conclude that no square circular entities exist in the universe.
But as for other conceptions of God, it really depends on what you're talking about. Everybody, it seems, has their own personal idea of what God is, and very few of them are willing to actually define what they mean by the word "God" before they go out bashing atheists for not believing in it. I am an atheist with respect to all the gods that people have defined for me. If they actually take the time to give a clear definition of their God, it pretty much always ends up describing some entity with logically contradictory attributes (i.e. God is omnibenevolent, but he's also willing to wipe out thousands of children in a tsunami), or it's just a bunch of gobbledygook that doesn't make any sense (God is ENERGY...oh, but not the physical kind Einstein was talking about that can actually be observed and measured).
