I was reading some correspondece concerning the question of agnostism versus atheism. The best discussion I have read is in George Smith's Atheism: the Case Against God. As my copy is on loan, I can only paraphrase.

Smith appels to the etymology of the terms "atheism" and "agnosticism". Atheism is quite clearly a negative term denoting a lack of theism. It's right there for everyone to discern A-Theism. Agnosticism is also a negative term and was introduced in the spirit of irony. The "gnostics" were ancient pagans who believed they were in direct communication with the spirit world. The term was later used to parody christians who claimed to have revelatory knowledge of the existence of god. An agnostic is someone who does not believe this knowledge is possible.

"Atheism" is concerned with ontology where "Agnosticism" is concerned with epistemology. Atheism is a passive lack of theistic believe (not an active belief in no god). Agnosticism is the belief that knowledge of the divine is not possible for human beings. It concerns the type of knowledge we can hope to gain, not whether or not we believe in god. I think that it is safe to claim without research data, that most atheists are agnostic-athiests.

When I was first researching rigorous atheism, Smith's book was one of my favorites. He really does a great job of explaining some difficult concepts.

I like the definitions you quote here, and I wish that they were universally used. Unfortunately, language is not so convenient so we have a variety of usages of these terms, adding to the general confusion. Is an agnostic someone who hasn't made up their mind, has decided that their mind cannot be made up, or who is an atheist that thinks proofs of the divine cannot be made? Is an atheist someone who rejects God, rejects gods, doesn't believe in gods, or isn't convinced that the divine exists? All of this highlights my love of defining terms before starting a discussion.