I disagree that agnosticism is not a possible position. I agree that atheists do not hold a default position. This is so because I see a difference between agnosticism and atheism such that the two are exclusive. I base this on the historical development of agnosticism. Under agnosticism whether we accept no god exists or a god exists is irrelevant. That is we can deny both or accept both but ultimately agnostics do not get to belief because they make the claim that we cannot, at least at this time, have knowledge to prove or disprove either assertion. Agnostics then remove themselves from the question. To understand agnostics all we have to do is play with cats in a box...is the cat alive or dead...most may extrapolate an answer based on acquiring knowledge of the situation. But given no other information, we are all agnostics. So too this is true with gods existence or lack thereof. We pile on evidences and make extrapolations however some believe that these evidences are not evidences at all. Some choose to disregard factors that might tip the scale one way or the other and simply say that we cannot know whether that cat is alive or dead. Whether they take the default position of non-committance or they assume the cat is both alive and dead, doesn't matter. The claim they make is that the knowledge we have about the subject has no bearing on the truth. With such a belief about our knowledge, the agnostics never rise to make a claim or exclude a claim. Understanding this, agnostics should not care whether they are considered to be at the "default position" or whether they are considered to believe both positions, because either form is descriptive of their views. Atheism however makes a claim that what theism claims is untrue.
I differentiate myself from agnostics because while I entertain the possibility of any god, I believe that theistic notions of a god are untrue. If however, we abstract the God concept in such a way that for all intent and purposes obliterated the theistic notions of god, gods, or God- then I would concede that such has a equal chance of truth as the materialistic or natural models of the universe. Or that we have no way of knowing, at this time, anything that belies the truth behind such notions. This however is irrelevant to the discussion about theistic gods and my beliefs about those theistic gods.
But, I can completely understand how one could, with a strong skeptic nature, not commit to any belief. Such a person however would be distinguished from atheists because they do not believe theistic claims untrue, but rather accept that a theistic god's existence is equally as true (or false) as its nonexistence.