Some time ago, I wrote page on why creationism lacks scientific merit. It was by no means a comprehensive analysis of the flaws in the theory, and it largely dealt with Young Earth Creationism. Intelligent design is an effort to bring creationism back as a scientific idea, and it is more insidious than YEC, but no less damaging to the proper teaching of science.
Over at Blogging Theology, I took part in a discussion on this very subject – I am no scientist and cannot claim to have a detailed understanding of everything underpinning either evolution or intelligent design – but neither can proponents of intelligent design.
Some proponents of ID try to deliberately distance themselves from the religious connection to the argument, in the hope that it will lend more credence to the idea – but every argument for ID can also be explained by evolutionary processes as well, without the requirement for an extra, undefined mechanism (namely the intelligent designer).
The key element here is the last part of the paragraph. The principle of Occam’s Razor teaches that the theory with the fewest definable mechanisms will always take precedence over a theory with undefined mechanisms, even if the latter theory has fewer mechanisms. Intelligent design relies upon a mechanism that cannot be defined (is the designer a supernatural deity, or an advanced alien race? If the former, it cannot be empirically measured, whilst if the latter, no such race has been observed or identified, and where did this species come from? How did it form?)
There are more detailed arguments that I could go into, but like I said earlier, I am no scientist, and there are other sites that can do this argument far more justice. I recommend Mike Wong’s excellent Creation Theory site as a starting point.