From The American Scientist, here's a concise and powerful statement of the reasons "Intelligent Design" is not science and why its presence in public schools should be opposed.
Allowing students to "opt out" of learning the basic facts and theories of biology is about as wise as allowing them to "opt out" of algebra or English: It constitutes malfeasance. [...] The ID movement is more than an attack on biology because evolutionary theory unifies the life and earth sciences with physics and chemistry. If ID is accepted as a credible science, then the most basic definition of a scientific theory and the fundamental principles of the scientific method are not being taught. [...] ID is an insidious attempt by a religious caucus to impose its views on the whole country. The avowed aim of ID advocates—to undermine science and replace it with their personal religious convictions—amounts to a form of prejudice that is both poisonous and horribly frightening.