In another debate with Bill Craig, atheist Christopher Hitchens gave this objection: "Who designed the Designer? Don’t you run the risk… of asking 'Well, where does that come from? And where does that come from?' and running into an infinite regress?" But this is an elementary misunderstanding in philosophy of science.

I agree that Hitchens should have looked to see what answers theists give to that question. (And he might have; since theists usually respond instead by saying that God is eternal, meaning outside of time and cause and effect, and therefore in no need of having a cause.) But I disagree that there are any more substantive objections to theism. "Who designed the designer?" is the best single knockdown argument against theism.

The question "where did God come from?" is not qualitatively the same as the question "how do you know your observation that a dropped bowling ball falls is correct?" In science, the answer to every "why" is something that is known with more certainty. Entropy decreases as you trace the epistemological/causal chain back up its causes. Theism, by contrast, boils down to the claim that entropy always increases as you trace back the causal chain. A being X must have been created by some being Y with greater entropy (complexity). The scientific epistemological chain converges; the theistic one diverges.

ADDED: This is basically the same as Tim Tyler's comment below.