In Who has the Burden of Proof? Atheism vs. Theism, Austin Cline (About.com) provides an outstanding discussion of something every atheist (and every religious person) should understand. Atheism (i.e., a lack of belief in gods) is the default position, the place from which everyone begins. This means that the burden of proof always lies with the religious believer who is claiming that some sort of god or gods exist.
The only claim being made by the atheist is that he or she does not happen to believe in gods. The atheist need not claim certainty that no gods could possibly exist.
The religious believer, on the other hand, is claiming that gods do exist. Such a claim requires considerable evidence if it is to be accepted rationally. Without such evidence, it is not rational to accept it as true. This is why appeals to faith are offered in the religious context.
It is not up to the atheist to prove that no gods exist; it is up to the person making the belief claim to demonstrate that gods do exist.