Agnosticism is, in fact, the least tenable theological position, completely inferior to atheism and in some instances less defensible than theism. Sweeney is correct that agnosticism for the most part constitutes a failure – and commonly a studied refusal – to think about the God question that much.
I had planned on writing something about agnosticism, but The Raving Atheist has done it better. Although his normal output is satire, his philosophical ravings amount to a body slam for the viability of agnosticism as an intellectual stance. You can't take agnosticism seriously if you really think about it.
Being agnostic appears to be a middle way, an intellectual demilitarized zone between theism and atheism. People profess agnosticism when they want to avoid having to go to church every week but they don't want their friends who still do to feel offended. Agnostics are closet atheists. I suspect that religious people prefer agnostics to atheists, much the way that many of those same people prefer homosexuals who stay in the closet.
Agnostics, consider yourselves "outed." The simple fact is that if you don't know whether something is real or not then you can't believe in it. Don't let the word atheist scare you; it's just a word for someone who doesn't believe in God, and it doesn't have any of the foul meanings that the religious want to associate with it. I don't know, and can't know, if there is a teapot orbiting Mars, so I don't believe there is. This doesn't make me an evil a-Mars-teapot-ist, just a regular sensible person.