One of the oft heard criticisms of Intelligent Design theory is that it is putatively non-scientific, i.e. it doesn't meet the criteria of an acceptable scientific hypothesis. Design or creationist theories have been alleged to be necessarily unscientific for a number of reasons: they do not explain by reference to natural law, they invoke unobservables, they are not testable, they do not make predictions, they are not falsifiable, they provide no mechanisms, they are not tentative, and they have no problem-solving capability.
Philosopher of science Stephen Meyer addresses the first three of these alleged short-comings in an article at the Discovery Institute's website. The essay is lengthy and somewhat technical, but it's an excellent analysis of the problem of trying to determine what constitutes science and what does not. In the philosophy of science this is called the Demarcation Problem, and, as many philosophers have noted, trying to find the boundaries of science is often counterproductive and is in any event a devilishly difficult task.
Before concluding that Intelligent Design is not science and Darwinism is, one should read Meyer's article. It would also be good, for the individual of a philosophical/scientific turn of mind, to read Del Ratszch's Science and its Limits and William Dembski's Design Revolution.