Up until recently, I, like many others, was under the impression that agnosticism was somewhere inbetween atheism and theism. That it was just "fence sitting". I learned that (a)gnosticism and (a)theism are not mutually exclusive when I watched this video by the Atheist Experience. Now I know that I am an agnostic atheist.
You can also read a great explanation about what agnosticism really is at the Atheist Blogger.