In a debate in Oxford, Richard Dawkins appears to have caused some surprise by saying that he wasn't absolutely certain of the non-existence of God. The assumption of his interlocurors was that in that case he should describe himself as an agnostic rather than an atheist.
I think this way of conceptualizing atheism and agnosticism respectively is mistaken: atheism doesn't require certainty; all it requires is that one thinks it highly improbable, on the evidence available, that God exists. Being agnostic is professing not to know one way or the other; an atheist takes the view that there is no God - on the basis of all that he or she does know. But this knowledge, like all knowledge, can be held probabilistically and subject to correction in the light of new evidence or fresh thinking.