Telic Thoughts has a discussion of the nature of "metaphysical naturalism" here to which I've submitted the following comment:
One of the fundamental problems in the debate over design is the vagueness of the terms "natural" and "supernatural." What exactly are natural or supernatural entities? Is a natural entity simply something which is part of the space-time universe and a supernatural entity something which transcends this universe?
If so, then those cosmologists wrestling with theories about "other worlds" are really doing theology, not science. If, however, we wish to consider the theorizing of cosmologists to be legitimate science then we have to say that excluding theorizing about an extra-cosmic designer from science is an arbitrary and unwarranted step.
For all we know, the designer could be a denizen of one of those universes or it could be the "generator" which manufactures those universes.
In other words, the concept of other worlds effectively erases the natural/supernatural distinction and greatly expands the purview of science.
The question then becomes not whether talk of a designer is scientific or not but whether there is reason to think that our universe and the living things in it show evidence of intention and intelligent engineering.
The next time someone tells you that Intelligent Design is not science because it invokes the supernatural and science only deals with what is natural ask them what they mean by those terms. Chances are they won't be able to give a coherent, non-arbitrary reply.