But Denny, you seem to be suggesting that "absence of evidence" is somehow a lesser or incomplete reason for disbelief. In fact, I'd say it is the #1 reason to "disbelieve" anything! You could tell me that you've discovered a miracle cure for arthritis based on magnetic bracelets and peppermint tea, but if you can't provide evidence, I'm afraid I will not believe you. Am I somehow "agnostic" on the possibility of peppermint/magnet remedies? "Agnosticism," in it's original definition, is a commitment to empirical evidence, and a refusal to hold as true that which cannot be objectively confirmed. It isn't a middle ground between theism and atheism -- it's a statement of knowledge, and how one should achieve it. The opposite of "agnostic" is one who believes that truth is achieved by personal revelation, rather than empirical and objective experiment.
You are correct that any true agnostic would immediately convert to theism if god rode a flaming chariot down to Earth and held a press conference. This would also not violate their status as agnostics. On the other hand, there are many theists who maintain that empirical evidence such as this is completely unnecessary for achieving true knowledge, that personal revelation through prayer or meditation is the best (or only) way to find truth. This kind of magical, subjective "truth" is exactly what Huxley was originally arguing AGAINST.
Now, the atheists.org position is that no theist can claim to be agnostic. I think that's a bit unfair, and disagree. But the bottom line is that agnosticism is not the fuzzy "middle ground" that has become the popular interpretation.