Interesting debate over at Shea's blog on materialism, atheism, and faith. The comments are particularly worth reading. A sample:

I am saying that you can, in fact, in practice, derive "ought" from "is."

In practice.

Certainly many materialists - in practice - behave as if "oughts" really exist. The thing is you can't - in principle - justify your practice.

Practice aside, there is no way - in principle - to conjure "ought" from "is" ex nihilo.The laws written on the human heart, to which Mark refers, are understood to be the unreflective moral principles upon which all moral reasoning rests. They are mysteries. Materialism can't account for them, or at least materialism can't give us any reason for following them.