How does atheism lead to objective morality?

That the question Douglas Wilson is hoping Christopher Hitchens will actually try to answer in their discussion posted at Christianity Today's web site.

So I am not saying you have to believe in the supernatural in order to live as a responsible citizen. I am saying you have to believe in the supernatural in order to be able to give a rational and coherent account of why you believe yourself obligated to live this way. In order to prove me wrong here, you must do more than employ words like "casuistry" or "evasions"—you simply need to provide that rational account. Given atheism, objective morality follows … how?