"Myth: The only logical basis for atheism is to know everything in other words, to be God![...]However, such knowledge is not necessary in order to make other negative statements. Examples of this latter type include "no married bachelors exist" or "no round squares exist." We can prove each of them with logic, primarily based upon the definitions of the terms involved. To assert the opposite of either statement entails asserting something which is incoherent and we are rationally justified in denying something incoherent. If an atheist believes that the statement "God exists" is similarly incoherent, then that atheist can say "no god exists" with certainty and without being ominscient."atheism.about.com, 20. September 2007
Some more reasons given. It's a must-read if you're unfamiliar with the issue.