For a believer it's different. They can logically make statements of certainty. I can't remember who posted it, but another member here put in some quotes from believers that sum this position up. I've spoken to many believers - in person, on the internet and read books - and have come across statements of certainty in god's existence many times. The reasons can vary. I've heard people say that they are certain of God's existence because "he speaks to me", "I feel his presence" or that they have actually met God (you don't have to spend too long on these boards before seeing that). The accuracy of these statements is irrelevant. People who claim to be in direct communication with god may be mistaken, but it's their subjective certainty that I was referring to.
In summary, if a believer states that they know beyond any doubt that god exists, this is not necessarily an illogical statement. However, an atheist cannot logically say that they know for certain that a god does not exist. In fact, I don't know any that have. Certainly not Dawkins.
I'm not sure I agree with you here, Arbie; I'm unclear why "it is impossible to exclude the possibility, no matter how small it may be" doesn't apply equally to a believer as to an atheist from a logic standpoint. I do think that both can 'logically' be absolutely certain in their beliefs if 'logical' is just confined to mean, 'proceeding logically/validly from the premises'. It is the premises that I think are the issue and are not wholly rational for some believers, and if believers can proceed logically from bad premises and still be considered 'logical', then I'm not sure why atheists should be excluded.
I do think you're right on point though concerning any atheists saying they know with certainty there is no god, I'm not aware of any but I wouldn't doubt they exist. I'm trying to come up with an example, it's difficult, perhaps someone 'believes' based on their understanding of science or physics that they've had some personal insight and believe that those laws definitively exclude anything but this physical realm. That would be a terrible premise of course, but I'm not sure how much better, 'my strictly internal, personal experiences confirm the validity of something in objective reality' is that much better for instance.