Intelligent Design's argument from ignorance

I'm very much struck by the observation in the blog linked from the title above that the bulk of the "Intelligent Design" (ID) creationists' arguments are actually arguments from ignorance. Basically, they say things like, "Nature is so complex; I can't see how this complexity could have occurred in the absence of purposeful design by some supernatural being." An argument from ignorance says that, since I don't know or understand something, it must be false. Clearly, this is nonsense: there are many things I don't understand, but they don't require my understanding to be true. The ID creationist argument is entirely equivalent to saying, "I can't imagine why anyone would want to fly an airplane into a building, and therefore 9/11 must not have happened."