Richard Dawkins has supposedly remarked that of all the arguments for the existence of a designer deity, the "fine-tuning" argument is most intriguing. According to some percentage of physicists, there's a very narrow range of physical constants that allow for life in the universe. If you take these physicists at their word (not everyone does), you're forced to either conclude we're here as the result of a hugely improbable coincidence, or there's some god-figure tweaking the controls.
Douglas Adams took on the debate. He imagines a puddle snuggled into a pothole, waking up and concluding that the universe was designed for puddles, since the fit is so marvelous. This is obviously an egocentric and deluded puddle, but it's a bit difficult for me to summarize exactly what Adams was trying to say. Adam's puddle seems to be pointing out that it's a fallacy to ever look at our existence and conclude that it was improbable.
But it does seem that some physicists, who don't show any signs of holding a god-proving agenda, believe that it is possible to ascertain that our universe is indeed an improbable one. That's not "improbable" in the silly sense of drawing 5 cards and then marveling at the odds that you drew that particular hand. It's "improbable" in the sense that the hand you drew is
one of just a few that could support life.
If we assume that our universe is indeed exceedingly improbable, does that point to a cosmic designer? You could also ask the same for replicators or consciousness or any other hurdle along the road to humanity. If we find that replicators are exceedingly difficult to design after 1,000 years of trying, what should we think? What if SETI never hears anything whilst probing the heavens? In my view, such scenarios don't improve the odds for a god-figure. Not much, anyway.
Why? Well, let's assume that our universe is indeed improbable. There are then two kinds of universes that are possible. One is the incredibly coincidental one in which conscious life arises (and inevitably marvels at the improbability of existence). The other one is a lifeless, chaotic universe, incapable of supporting simple molecular bonds or any kind of complexity at all. But the mindless plasma in such a chaotic universe never gets the opportunity to consider the ordinariness of its situation. The things that are capable of pondering existence only arise in improbable universes.
So, if you have a near-infinite number of universes, you're bound to get some conscious life every now and then. But what will that conscious life do? It will do physics and calculate that the universe is improbable and must therefore be designed. The conclusion of improbability would be correct, but the conclusion of design would be incorrect.
There's no argument for a creator if multiple universes are allowed. However, let's assume that 1) our universe is indeed improbable and 2) it's the only one that has ever been. What should we then think? I'm not sure.
It should be pointed out, again, that not all physicists have decided that life can only arise in a very small fraction of possible universes. Victor Stenger argues that a large fraction of configurations of the constants of physics still allow for life. But then he goes on to make the case for multiple universes, saying that they don't violate Occam's razor (as creationists have argued). So he seems to be covering both bases, saying that life is not improbable, but even if it is there's no incentive to lean toward a creator. Nothing wrong with covering both bases.
You could almost turn the fine-tuning argument on its ass and use it to argue for the existence of multiple universes. Like this:
1) There's no Cosmic Designer (see Dawkins, Hitchens, Harris, et al for thousands of pages devoted to this topic).
2) The universe is improbable.
Therefore, there must be multiple universes.