There's an interesting article on Slate by someone miffed at all those 'correlation doesn't impy causation' slams common in retorts. He makes the good point that, while not proof, correlation is suggestive, and consistent with causation. If you want to be a pedant nothing non-tautological can be proven (see Hume's problem of induction). It reminds me of the saying 'absence of evidence is not evidence of absence.' This is wrong. It's not proof of absence, but from a bayesian perspective, it should increase one's belief in absence.