It seems to me that there are at least four different ways we might characterize Intelligent Design theory.
1. As an alternative to Evolution. On this view ID folks would bear more or less the same relationship to Evolutionary biologists as Copernicans did to Aristotelian astronomers in the Sixteenth century.
The problem with this characterization is that Intelligent Design theory of itself seems too vague and minimalistic to constitute a real alternative to evolution. All ID folks will say is that life on this planet is a result (at least in part) of some intelligent force or being(s). They deliberately will not say what the nature of this force is (God? aliens? time-travelers?), nor do they have much as a group to say about how this force accomplished its task (was it done ex nihilo? did it happen instantaneously or over time, and if so how much time? was it done directly or via some mechanism, and if the latter, what is the mechanism?) Some ID advocates are willing to accept large swaths of evolutionary theory (Michael Behe, for example, is willing to accept the common ancestry of all living things) others aren’t. Without saying more, it’s hard to see why ID theory couldn’t even be compatible with Evolution.
2. As an (un)friendly critic of Evolution. We might construe the ID folks as offering criticism of Evolution without offering any alternative. So, for example, current evolutionary theory can’t account for irreducible complexities like blood clotting, and so needs to be modified or abandoned. Decent based solely on natural selection and random mutation can’t account for the fact that our rational faculties are reliable, so can’t be the whole story, etc. Such a person might think that the theory can be suitably modified so as to be salvageable but not know how to do it, or he might think that the theory should be scrapped but not know what to replace it with. In either case, though, his criticisms will be entirely negative.
The problem with this view is that ID theory quite clearly does go beyond simply criticizing Evolutionary theory – it suggests that the solution to these problems lies in the idea of intelligent design. Thomas Nagel, for example, has offered criticism of Evolutionary theory and seems to suggest that it ought to be scrapped. But he also disclaims any belief in intelligent design. I take it that, as big a tent as the ID movement is, Nagel’s views are beyond it.
These considerations lead naturally to
2a. Modified critic of Evolution. According to this view, ID theory not only shows us the holes in Evolutionary theory, it shows us how to patch them. More specifically, it suggests that various problems in the theory can be solved by modifying it to include reference to intelligent design. Different ID advocates will differ as to the extent of modification needed, but all agree that it is needed and that the best solution involves postulating intelligent design.
This is perhaps the most plausible of the characterizations, though I suspect that some ID advocates would draw back from the claim that there is anything salvageable in evolutionary theory. Still, this characterization does have a rival, namely
4. As an Interpretation of evolution. On this view Intelligent Design theory is opposed not to evolution, but to the interpretation of evolution given by methodological naturalism. The ID theorist might claim to have no beef with the fact of evolution (descent with modification, etc.), only with the claim that this process was unguided. This, he might claim, is not a implication of the scientific evidence, but rather a philosophical add on. The difference between ID and Naturalistic Evolution on this view would be similar to the different between the Copenhagen interpretation of quantum mechanics, and the many-worlds or hidden variable interpretations.
Why does it matter? Well, because clear thinking is always a good thing, for starters. But also because how we characterize ID theory is going to affect whether and to what extent we think it ought to be taught in the schools.
Waiving for the moment any Constitutional difficulties, it seems quite plausible that we should be teaching the best and most useful scientific theories. So if 1. is right, then whether we should teach ID depends on whether it is a better theory than evolution. If it is, then it should be taught and evolution shouldn’t be; if it isn’t, then evolution should be taught and it shouldn’t be. (there is a place in science courses, especially the more advanced ones, for discussion of discarded or otherwise lacking scientific theories, but in my experience a comprehensive presentation of those theories is rare). I would think, also, that we ought to defer by and large to the scientific community in determining which is the better and more useful scientific theory. So, ironically, construing ID in this way might lead to its not being taught at all.
If we take view 2. or 2a. the matter is somewhat different. There may be slightly more of a place for discussing problem areas in a theory than for discussing worse-off alternatives (and, of course, if we think that evolution is fatally flawed but have no theory to replace it with we might not want to teach the subject at all!). Still, I would imagine such discussion would be quite limited.
What if we take characterization 4? In that case, it would probably be worthwhile to mention the disagreement, but a high-school biology course is not a philosophy of science course, and probably shouldn’t be. A full fledged discussion of these issues would then be inappropriate.
The upshot (downside) of all this is that unless we take view 1., it doesn’t seem appropriate to teach Intelligent Design theory in more than a minimal way even on the assumption that the theory is true and that there is no Constitutional impediment to doing so. And assuming view 1. may lead to the theory’s not being taught at all. Which is somewhat surprising.