In online debates about religion and the existence of God, there is a disturbing trend. Internet atheists often say that "there isn't a God" isn't a real claim and the only way that they will believe in the existence of a god or any other deity is for the theist to definitively prove his existence. They often will say that the burden of proof doesn't fall on the atheist, but the theist.
This is wrong in two ways. First of all, claiming that there isn't a God is a claim, it may be a negative claim, but it is still a claim, just like "The Earth isn't flat" is also a claim. It may be a negative claim, but nevertheless, it is still a claim. You are making a statement about the world and how it is, that is, in fact, a claim whether you like it or not. If you are making a claim, then the burden of proof falls on you also. Just as the theist must prove that there's a god, the atheist must find proof that there is not a God. Atheists who claim that atheism lacks the burden of proof are just cowards who are too lazy to come up with a good argument against the potential existence of a creator, or they are afraid of being proven wrong.
Also, atheists tend to reject evidence and arguments that theists present to them for the existence of a god. They usually reject it on the grounds that the evidence isn't scientific enough. The problem with this though is that the existence of God isn't a scientific argument at all. There is no empirical data proving or disproving the existence of a god or gods. God isn't a scientific concept at all. The concept of God is may differ from person to person, but God is supernatural and not scientific. There is no scientific experiment that a person can devise to prove or disprove the existence of a deity, and this may bother the scientifically-minded, but there simply isn't much that can be done in that realm.
While there is no definite evidence of God, absence of evidence is not evidence of absence. There may or may not be a god, but if you are making a claim, the burden of proof does fall on you.