Believe it or not, most atheists aren't trying to prove anything. We are simply unconvinced that the available evidence points to God. For such a truly monumental claim to be accepted, it seems like there out to be a more convincing reason to believe. Or even an irrefutable reason to be believe, one left intentionally for us to find. That's all.
As to proving a negative, it may not always be true that this can't be done. But in this case, I think someone did a good job of illustrating our point (where we feel the burden of proof is on the believer) when they asked everyone to prove the existence of a historical figure.
There are other scenarios. Can you truly, positively prove to me that there are no unicorns? Maybe I think they're invisible, or they dwell in a place that man has never been to, or that there's a conspiracy to conceal their existence. Or maybe I think they were taken by aliens without a trace, so we will never find them unless the aliens bring them back. You would never be able to positively, absolutely prove they don't exist to me if I wanted to believe in them in the first place. Asking an atheist to prove God doesn't exist is a lot like that from my point of view.
But it seems like there should be evidence for a theistic god and we haven't found it yet. Some measurable, objective signs of design or evidence of its wishes. Unless it's an evil god that is merely toying with us and trying to confuse us, but why would it bother? None of the proofs we have seem to be very good. The Bible, for instance, just doesn't cut it for me. Not even close. And it seems that a natural process is at work here rather than an ordered, superhuman intelligence, which explains why human intelligence is enough to point out the design flaws.
A deistic God is another matter. But I don't see the point of believing in one, especially since I don't see how such a being matters very much. Why would I just assume that such a being is out there any more than I would accept the existence of unicorns without good, solid proof? Doubt seems like the default position with such a fantastic claim.
Note that everyone is free to believe what they want. I'm only explaining a little about my view points. I'm not arguing with anybody here. I'm already sure most of the believers completely disagree. These are just some of the reasons I disagree with them.
BTW, Dawg asked me about the articles I'd read regarding artificial life in a lab, and I can't find either of them now. So unless I can find them again, consider that a case of "Isaac speaking too darn quickly." I suck. Sorry about that. My bad.
IsaacJ