In an actually useful HuffPo post, Michael Shermer discusses intelligent design, offering an updated version of Philo's objections in Hume's Dialogues. Namely, if the best explanation of various phenomena is design, then we require a theory of the designer. And the best theory may simply be a committee of super-intelligent but fallible aliens. Which, clearly, get us no closer to the God of Abraham than we were before.
Now, Cleanthes, said Philo, with an air of alacrity and triumph, mark the consequences. First, By this method of reasoning, you renounce all claim to infinity in any of the attributes of the Deity. For, as the cause ought only to be proportioned to the effect, and the effect, so far as it falls under our cognizance, is not infinite; what pretensions have we, upon your suppositions, to ascribe that attribute to the Divine Being? . . .
Secondly, You have no reason, on your theory, for ascribing perfection to the Deity, even in his finite capacity, or for supposing him free from every error, mistake, or incoherence, in his undertakings. There are many inexplicable difficulties in the works of Nature, which, if we allow a perfect author to be proved a priori, are easily solved, and become only seeming difficulties, from the narrow capacity of man, who cannot trace infinite relations. But according to your method of reasoning, these difficulties become all real; and perhaps will be insisted on, as new instances of likeness to human art and contrivance. . .
And what shadow of an argument, continued Philo, can you produce, from your hypothesis, to prove the unity of the Deity? A great number of men join in building a house or ship, in rearing a city, in framing a commonwealth; why may not several deities combine in contriving and framing a world? This is only so much greater similarity to human affairs. By sharing the work among several, we may so much further limit the attributes of each, and get rid of that extensive power and knowledge, which must be supposed in one deity, and which, according to you, can only serve to weaken the proof of his existence. And if such foolish, such vicious creatures as man, can yet often unite in framing and executing one plan, how much more those deities or demons, whom we may suppose several degrees more perfect!
Or, try this. ID, even if true, puts us in an explanatory spiral, an unclosed regressive loop.
Assume ID is the best explanation for ordered complexity. That means, our best theory of ordered complexity posits the existence of an intelligent designer, meaning that we posited intelligence as an explanatory fundamental. However, intelligence as we know it is a property of biological beings, and a form of the kind of ordered complexity we initially sought to explain.
If it is suggested that “higher” intelligence is not a form of ordered complexity analogous to our own intelligence, then there is no ground for calling it intelligence after all. If it is itself a form of ordered complexity, then we have made no explanatory advance, for we will be left positing an even higher order intelligent designer for each higher order intelligent designer.
If it proposed that we stop the explanatory spiral by positing an undesigned designer then a new question arises: What explains the emergence of the undesigned designer? Whatever the explanation for the ordered complexity of the undesigned designer may be, then it seems that that explanation could be applied to first order ordered complexity, and Occam demands we excise the useless proliferation of higher order designers.
If it is replied that there is no mechanism that gave rise to the undesigned designer, then first order ordered complexity is still unexplained, only it is now more elaborately unexplained.
Even if it's the best explanation, ID would get us nowhere, which means its probably not.
My take on ID is that if there were any evidence for it, then the probability of intelligent extraterrestrial life would be non-zero. We would then have a proximate explanation for ordered complexity as it appears on Earth. But we'd be no closer to an account of ordered complexity as such.