Plantinga received a research grant from the Ammonius Foundation to write on the subject of naturalism and objective morality. The completed paper is out and published in Faith & Philosophy (vol. 27, no. 3 (2010)). If you are interested you can find Plantinga’s paper entitled Naturalism, Theism, Obligation and Supervenience by clicking the link. I haven’t had the opportunity to read the paper completely but Plantinga argues that naturalism does not accommodate moral theorizing.