James Chastek has a good post on why Cosmic ‘Fine Tuning’ arguments are unsatisfactory:

Fine tuning arguments play on one of the great blind-spots of modern thinkers- the muddling of logical and real possibility. The arguments can only swiftly reach a conclusion because we think we can go from imagining things happening in a different way to concluding that there is a real possibility that they could have been so. Descartes famously  muddles these two when he assumes he must take an evil deceiver as a real possibility (indeed the true original sin of modern thought is not “how do I know that I know?” but the more fundamental error of identifying logical and real possibility). Again, Analytic philosophers – especially the theists – are prone to make the same sort of mistake (Plantinga’s argument for mind-body dualism is a good example; so is the popular Analytic claim that God exists because he is possible; and  in general the interest one takes in the ontological argument is proportionate to the degree to which he muddles the diverse kinds of possibility… and talk about “possible worlds” is a category of its own in this confusion…)