American Prospect Online has this brief but interesting history of anti-evolutionism in America and a good description of the role that "Intelligent Design" plays in the whole mess.
ID is an idea with a respectable philosophical pedigree, though no credibility in modern science. The basic notion hinges on the organizational complexity of living organisms, and especially anatomical marvels such as the eye. How could evolution have done that, ask ID proponents?
The answer, provided by Darwin and repeatedly supported by scientific investigations since then, is that given enough time and selection pressures, evolution tends to find a way. Moreover, its solutions, while certainly workable, are not always ideal. As anyone with any experience of aging knows, the human body isn't a paragon of perfection. Its flaws and rudiments -- the appendix, male nipples, and so forth -- seem far more characteristic of the mindless tinkering activities of natural selection than of an intelligent craftsman.