Great question (and great response by Allen). Let me just add a tiny bit, by encouraging you to check out both Norman Malcolm's and Alvin Plantinga's work on the ontological argument. (The latter is a lot more technical and difficult, so start with the former.) From them you get something like the idea that if God exists at all, He exists necessarily (for God surely isn't a contingent being); to say that God exists necessarily is to say that He exists in every possible world. But now, if it's even possible that God exists -- i.e. the idea of God contains no contradictions -- then God would exist in at least one possible world. But if He exists at all He exists in every possible world, so if He exists in one PW He exists in every PW. Now is it possible that God exists? Does the idea of God involve any contradictions? Lots of discussion in the history of philosophical theology on that topic (lots of purported contradictions posed, then response to), but lots of people, even many ordinary atheists, think there's no contradiction in the idea of God, just merely that God contingently doesn't exist. So if you construe the multiverse theory to mean that every possible world exists (not sure it should be construed this way, but let's suppose), and if you think the idea of God involves no contradictions, then it sounds like the multiverse theory could support this line of argument toward God's existence.
hope that's useful!