In 2008, Alvin Plantinga got a research grant from the Ammonius Foundation to write a paper on naturalism and objective morality. The paper ("Naturalism, Theism, Obligation, and Supervenience") is now out in of Faith & Philosophy (vol. 27, no. 3 (2010)). Here is a link to the paper.
I'm too busy at the moment to give it a careful read, but it looks as though it employs the Common Apologetic Strategy, and thus falls prey to the problems inherent in such arguments.