The Ad Hoc Multiverse
To be honest, postulating parallel universes strikes me as a desperate move, a textbook example of a Kuhnian, ad hoc supposition to save Naturalism in the face of powerful evidence to the contrary. To be sure, theism entails its own fantastic theses, but perhaps none as far-reaching and unsubstantiated as the multiverse. There are some smart people who believe in parallel universes, even some physicists who believe that in quantum physics we can detect interaction with a parallel universe. But even if we grant that this highly contested interpretation of quantum physics has merit, and grant that this interaction should not be understood as a mysterious feature of our own universe, this "scientific" evidence falls far short of establishing the panoply of universes required to undercut arguments for a Designer.
When faced with the fine-tuning of this wonderful universe of ours, if Naturalists make the move to a multiverse hypothesis, they bely a curious prejudice for things made of matter. The multiverse may be untestable, unobservable, unverifiable (sound familiar?) ... but at least, in theory, it's made of stuff. They are also making a noteworthy, if unintentional, concession. In turning to such a radical solution, there is an admission that in this universe there is, in fact, a striking appearance of design, or at least a colossal improbability, that demands an explanation. Our universe cannot be explained from within itself.
Update: Since originally posting this clipping, Discover Magazine has published an excellent article by Tim Folger on multiverse theorizing: "Science's Alternative to an Intelligent Creator: the Multiverse Theory". In quite poetic prose, Folger states the problem well and surveys a number of efforts in theoretical science to substantiate the multiverse.
Physicists don’t like coincidences. They like even less the notion that life is somehow central to the universe, and yet recent discoveries are forcing them to confront that very idea. Life, it seems, is not an incidental component of the universe, burped up out of a random chemical brew on a lonely planet to endure for a few fleeting ticks of the cosmic clock. In some strange sense, it appears that we are not adapted to the universe; the universe is adapted to us. ¶ Call it a fluke, a mystery, a miracle. Or call it the biggest problem in physics. Short of invoking a benevolent creator, many physicists see only one possible explanation: Our universe may be but one of perhaps infinitely many universes in an inconceivably vast multi verse. Most of those universes are barren, but some, like ours, have conditions suitable for life.