The religious like to say that the absence of proof is not proof of absence. They are being disingenuous when they say this, however. They are deliberately confusing the argument by their choice of words, substituting "proof" for "evidence" in this saying: The absence of evidence is evidence of absence.
It should be clear that both of these sayings are generally true even though seemingly contradictory. The reason for this apparent dilemma is the subtle difference in meaning between the words "proof" and "evidence". When one uses the word "proof", a secondary meaning, "conclusive proof", comes into play. The word "evidence" does not imply the same level of conclusiveness.
When there is no evidence for a thing, that lack does tend to prove that the thing doesn't exist and thus is "evidence" of that fact--the question of the conclusiveness of the evidence being a separate issue.
Thus, when the religious bring this up and use the word "proof", they are saying that the absence of evidence doesn't conclusively prove the atheist's point, but, more important, they are trying to confuse you regarding the fact that the absence of evidence does, in fact, tend to prove absence. When you hear this type of argument (no matter how it is phrased--they might get it wrong), simply say:
"The absence of evidence for a thing is evidence of its absence. Whether or not it is conclusive evidence is another issue."
Or, you can say:
"I think what you mean to say is that is doesn't conclusively prove absence. It does, however, tend to prove absence."
As I will explain in a later post, in the case of the god hypothesis, the lack of evidence is sufficient.