Well, it's a fallacy, called in Logic as argumentum ad ignorantium - we say that something is false because we can't (or don't) prove that's true. So, in this case, would be the same thing for theists and atheists: anyone who use this kind of argue would commit a fallacy.

But we have other thing to analize in here: the burden of proof. Ok, if we use logic, for the atheism exist, theism had to be around first - off course, we can't deny something that doesn't exist (in this case, the idea of god's existence, and not the god itself). So, who has the burden of proof are the theists, and not atheists. They had to prove that god exists, and not the opposite.

My conclussion: to get out of this labirint, theists have to prove that god exist. But if it happens, and i really doubt it, other problem will appear: faith. For definition, faith (from latin fidis) means "to believe in something with no prove". If there's prove, then the base of all theists will not exist. And we will have another philosophical problem.

P.S.: Sorry for my "not that good" english.