There's an argument against Intelligent Design creationism that I've not seen anyone make yet. (It's implicit here or there, but I've not seen it articulated as a separate argument.)
Most of the arguments against Intelligent Design have, rightly, focused on the fact that it's bogus -- that it's claims are either untestable and unfalsifiable (and hence not scientific at all) or are claims which have been tested and falsified already. This seems contradictory, incidentally, because Intelligent Design itself is nothing very coherent: it makes a grab-bag of bad claims of all sorts: unfalsifiable metaphysical claims, simple lies about science, presenting specific outstanding scientific questions (which there are of course, that's how science works) as if they somehow falsified evolutionary theory as a whole, deceptions based upon misunderstandings about the nature of science (i.e. what the word "theory" means in a scientific context), etc, ad nausuem. These are the main arguments made because it's the key point: ID's simply bogus (I prefer the word "bogus" because it covers both "wrong" and "unfalsifiable" and "deceptive" -- the various different ways various ID claims are, well, bogus.)
The other major argument -- about why the "teach the controversy" position (recently adopted by Bush) is wrong -- focuses on the fact that ID is a trojan horse. It's not science, but an attempt to teach religion in science class as if it were science. (No one opposes teaching religion in classes about religion, as long as it's not a single religion being promoted.) The key thing to understand here is that ID was engineered -- quite deliberately -- to work around the 1987 Supreme Court decision declaring the teaching of "Creation Science" to be a violation of the first amendment's disestablishment of religion: don't mention who the designer is (though everyone knows, nudge-nudge, wink-wink), hide your religions motivations (although they come out when ID's proponents let their hair down), and so forth. This is why ID shouldn't be taught in science classes: it's teaching a religious point of view in badly-fitting secular drag.
But I think there's another argument to be made as well. Assume for the sake of argument that ID is what its leading proponents say it is: a scientific challenge, made on scientific grounds, challenging evolution for purely scientific reasons and not ulterior motives. (Just to be clear: this is what philosophers call an assumption-contrary-to-fact, i.e. it isn't true; we're just assuming it is for the sake of this argument.) Even given this assumption, ID still shouldn't be taught in public high schools -- to say nothing of junior high schools, etc., nor in basic biology classes at the college level. (This particular argument doesn't hold for advanced classes (though of course the others do.))
Why not? Because the point of basic science education -- such as that of junior high, high school and introductory college classes -- is to teach basic science (and the scientific method, etc.) A fringe theory, one not yet accepted broadly in the scientific community, simply has no place there. Sure, this means that years after a student takes their classes, some of what they were taught will have been rendered obsolete -- but that's because of what makes science good, namely that it progresses. But since we can't know in advance where science is going, what basic science classes must teach are the best science of the day -- not fringe theories that have yet to gain acceptance.
This is obvious if we look at any other scientific controversy. I believe that there are still scientists who are promoting the idea of cold fusion -- but I'm sure that they themselves would agree that it shouldn't be taught in basic science classes until it gains wider acceptance. Certainly no one else would think so. Similarly with any other minority theory -- including ones that have come to be widely held, back when they were fringe theories: for a time the notion of continental drift was accepted only by a few -- and no one thought it should be taught in high schools; only when the evidence became overwhelming for it did it enter the basic curriculum.
Of course, in some cases a dispute is too live and too central to ignore. For instance, at least the last time I checked (if anyone has any more up to date info, email me) there was an ongoing question about human evolution, whether humans evolved into essentially their present-day state in Africa before spreading out into Asia, Europe and elsewhere, or whether earlier forms of primates spread out, and modern day humans evolved (as it were) all over, with enough interbreeding to keep us all one species. (I've never understood how the second makes sense, so perhaps I'm explaining it wrong. But of course it doesn't matter what I think; the vague understanding of laymen is not what matters in science, as is sort of the whole point here.) So if talking about human evolution, perhaps you'd have to mention both theories, at least until the evidence becomes clearer. But of course ID is not any such theory. The overwhelming majority of scientists reject it. It hasn't even published any real papers in peer-reviewed journals (except for one review essay shoved in against journal policy by a partisan editor.) So even if you (as per our contrary-to-fact assumption) take ID to be simply a scientific movement, there is absolutely no reason to teach it in high schools. It's just too fringe. Wait until it becomes a major movement, with serious scientific support, and then think about teaching it. If ID people were serious scientists, they'd agree -- no rush, wait until the evidence is in and more scientists become convinced. This, incidentally, is why ID proponents always talk about a growing controversy: to make it look like it's winning, not only for Rovian-people-join-the-winning-side reasons, but so as to make the idea of teaching it in schools look a little less ridiculous.
So why do people want it taught? Here, alas, is where our contrary-to-fact assumption breaks down (as they tend to do, since they are, well, contrary-to-fact). People want it taught for non scientific reasons -- specifically, for religious (or political and religious) reasons. Its major proponents -- Philip Johnson, Michael Behe, William Dembski and the like -- don't want to wait for more evidence to roll in because they're not waiting for evidence at all. Maybe it's because they're insincere, and know that what they're promoting is bunk and will never get more evidence. But even if they sincerely believe what they claim (and who knows, really), they are pushing it into schools for reasons other than those (supposedly) scientific reasons. If they simply wanted to get to the scientific truth, they'd wait: all sorts of developing science isn't taught in basic science classes, if it catches on it will be, until then what's the rush? But they want to undermine what they see as a tool of atheism -- evolutionary theory, and a materialist worldview more generally. (They've been explicit about this from time to time, although they try to hide it since it damages their strategy of appearing neutral for legal reasons.) Their goals, in other words, are religious, philosophical and political -- not scientific; and this is true even if they believe what they claim.
(Now, as I understand it, the current position of the Discovery (sic) Institute is that ID shouldn't be taught -- so to that extent they're honest, or at least trying to appear so -- but that the "problems" with evolution should be. But their supposed problems with evolution are as bunk as their notions on ID -- and the same argument applies: if they were serious about this as science and only as science, they wouldn't want these arguments taught until they'd gained wider acceptance. Pushing the teaching of so-called problems with evolution is simply one form of pushing the teaching of ID. For that matter, all that ID is is a bunch of bogus arguments against evolution -- there's no positive program there at all (as its proponents will also admit in unguarded moments.) So despite their attempt to appear fair, DI really is pushing for ID to be taught in the schools.)
That's the central proponents, most of whom are associated with the Discovery Institute. (Dear Lord, the very fact that a think-tank is the center of a purportedly scientific movement should be a dead giveaway!) But for the majority of proponents -- school board members, politicians, preachers and sundry other citizens who are pushing for ID to be taught on a local level -- it's even clearer. After all, most of them believe, frankly, in creationism. And therefore they're even less disciplined about hiding their religious motives than the central proponents are. (That's why the Discovery Institute is discomfited by the case in Dover, Pennsylvania: the Dover school board was too open about its religious motives, and thus are hurting the essentially deceptive legal strategy of ID.) Probably some of these other ID proponents don't understand the legal background and thus the necessity to hide their religious motives; probably others feel that they oughtn't to have to hide them and are frankly in favor of putting partisan religious views into public schools, the first amendment be damned (literally, I suppose). Whatever the reasons, though, they are far more open about it -- they will talk about teaching "divine design", for instance, which really messes up the ID/DI legal strategy, or openly say they are for ID because they believe in creationism, which does so even more. In other words, they see ID as essentially equivalent to creationism -- legally acceptable, but basically the same thing. (So at least on one issue, they're right.) Which is why they want it taught.
The very fact that people are pushing for ID to be taught in public schools shows that their motives are not scientific but religious (and/or political). The very fact that people are pushing for ID to be taught in public schools shows why it shouldn't be.