Now, I certainly agree that the theist must provide evidence. That's why a failure to disprove the existence of God isn't really a point in favor of theism: there's still the work of showing that there is a reason for belief. But should we be concerned about those theists who don't see it that way – who regard the failure of an argument against God as evidence for their side? I would say that if we do, then we must counterbalance that against any benefits associated with arguing in favor of positive atheism.
My impression is that Matt doesn't think there are any such benefits. If so, that's where we disagree. I think it is worthwhile to argue in favor of positive atheism, for at least two reasons. First, putting the opposition on the defensive is a good thing. If we take Matt's approach and merely present objections to the theist's arguments, we may succeed in keeping the burden of proof on their side, but as far as most theists are concerned, we have at best shown that they need to come up with better arguments. They will not feel that we have defeated their conclusions but at most the method they use to try convincing nonbelievers. If on the other hand we give them arguments for positive disbelief that they cannot answer – well, in that case, they have a real problem!
Second, our main concern – and I assume Matt will agree here – shouldn't be with winning arguments but with finding the truth. Discovering good reasons for disbelief – or better yet, conclusive reasons – is therefore very worthwhile. I think there are several strong arguments for disbelief and at least one conclusive argument that shows no deity (on the traditional meaning of that term) can possibly exist. All of this is discussed in The Truth about God.