Just a few thoughts I thought I'd air here.
Is GMR contingently true? So far I've seen nothing that would indicate that if there are a plurality of concrete worlds there are necessarily a plurality of concrete worlds. Consider a Lewisian universe in which all worlds with green rabbits aren't there. That seems just as metaphysically possible as the universe in which all Lewisian worlds are there. If that's true, any given Lewisian world is a contingent entity (the fact that he lacks the vocabulary to describe what that would mean irregardless). In other words, most actualist possible worlds exist in virtue of the world possibly being a certain way (and thus all possibilities are necessarily represented). Lewisian worlds don't have this to fall back on.
utterly contingent.... disgusting!
Also, I had a couple chats with the reductionists of the class about the epistemology of modality. The question had come up, "how can we know about possibilities". This could be directed to either realist theory.
Well, I'll attempt an empiricist approach to this problem. We can observe actual properties and relations and conceptually divide them from their instances. We can also notice scientific trends, or natural laws. If we can conceive of a few properties and relations combined, and notice that nothing in our natural laws prohibits this combination, voila! We've conceived of a possibility. Of course hard determinists will have to be more strict about the laws they employ, and give a story about a causal history. But that's ok, we don't need to be hard determanists (at least no present natural law entails HD).
If we grant Lewis that all possibilities are realized in a concrete world, then voila! We know stuff about other concrete worlds. However, if all these worlds are contingent, we have less cause to grant then to Lewis.