Prosthesis has put into words what I've been trying unsuccessfully to say for quite a while now. Intelligent Design is technically philosophy and not science, but it's no as simple as that. It's the most recent form of the teleological argument for the existence of God, in the tradition that began at least as far back as Aristotle, something like 2500 years ago. For him, of course, it was natural philosophy, which has become the science of today, so it's not as if it's outside the realm of what we now call science, but that argument has remained in philosophy as most other subject matters of Aristotle's natural philosophy have become known as science.
Intelligent Design is certainly not religion. That's evidenced by the people who support it who do not believe in any religion. Antony Flew is just the latest and most famous proponent of ID not to accept any religion as a result of it (or even many of the traditional attributes of God). Michael Denton was saying similar things in the 1980s. He's an atheist, and he thinks some sort of argument along the lines ID people give is correct against the standard neo-Darwinian picture (though he accepts large parts of it, as do most IDers). It's true that most ID people are Christians, but that doesn't mean it's religion, either in principle or even for those Christians who accept it. It's a philosophical argument that many Christians accept, and they accept it because they think it's a good argument, not because it occurs in the Bible.
People like Brian Leiter and most of the people at Panda's Thumb show their philosophical ignorance in saying it's an imposition of religion, and that shows how radically ignorant they are, but even a more moderate ignorance is, to my mind, culpable, and many of my philosophical colleagues exhibit it. Their denial that ID is science seems to me not simply to be an agreement with my claim that ID is philosophy. It seems to me that it's often an unwillingness to recognize that philosophical arguments of the same caliber as the ID argument often play a role in good scientific reasoning. Anyone even remotely familiar with the contemporary literature on philosophy of science should think this obvious. Just about all scientific theories involve metaphysical assumptions, and they rely on philosophical arguments for those assumptions.
This is especially so for physics, and the ID argument that I think has the most going for it is the cosmological constant argument for a designer, which falls in the realm of physics in the same way most of what Stephen Hawking has popularized falls under the realm of physics, despite the fact that most of what he does in those books is philosophy (and not always very good philosophy, certainly not any better than the ID argument based on cosmological constants). I've looked at Stephen Hawking and the responses from the ID side by people like Hugh Ross. It all seems to me to be doing philosophy as a part of the scientific endeavor. To say that ID is not science is thus to limit science to a small portion of what scientifists in fact do.
I think Macht's post is especially helpful in pointing out that this is so for people like Stephen J. Gould and Richard Dawkins. The kind of general theorizing that they do is basically philosophy of biology. The kind of general theorizing Stephen Meyer does falls in the same category. If his ID work isn't science, then neither is their work in evolutionary theory. Both are working with the philosophical elements of science, but I think it's right that they're equally science if those elements of evolutionary theory count as science at all.
As I've said before, I'm not going to do any evaluation of Intelligent Design (in the biological form) or of evolutionary theory on this blog. I don't know enough biology to engage in that enterprise. I don't consider someone qualified to judge these arguments without knowing a lot more biology than I and most philosophers do and without knowing a lot more philosophy than most biologists do. I might do a post this summer on the physics argument for a designer, since I'll be teaching that material again this summer for the first time since 2003. I know something about that and have some sort of opinion about its import. My point here is simply that the claims I hear over and over again from people who should know better, to the effect that ID is not science, almost always seem to me to be coming out of ignorance either of what ID is (and see the parts of Macht's post that clarify the six different senses of 'evolution' for some clarity on that) or ignorance of what science usually involves.
I hadn't quite thought to put it the way Macht did, but I think that final thesis is correct. ID arguments are the sort of thing that can find their place in science given what other sorts of argument commonly find their place in science. I've said all along that I think they should be taught in high school in philosophy classes, which I think should be mandatory for all seniors, but this explains why the argument should be taught in biology classes too. All most proponents of ID want is that the argument should be taught. Biology teachers, if they so choose, can teach about how they think it's a bad argument (as most of my colleagues do when they teach it in introductory philosophy classes in college), but that's teaching the argument. It amazes me that there's such resistance even to that. It makes me wonder why the opponents of ID wish to be so intellectually dishonest as to pretend something so ridiculous as that it's mere religion just to avoid something so limited as presenting the argument and then criticizing it.
Update: David Heddle presents some further evidence of the intellectual dishonesty of those opposing the teaching of biological ID. It's completely irrational, and the sad irony is that these are the people accusing IDers of intellectual dishonesty. There aren't too many examples of outright, undeniable hypocrisy nowadays given the ease of doublespeak and redefining of terms to avoid such criticisms, but this one is pretty hard to get around.