Since there's been a lot of back and forth about what a "Scientific Theory" is, let me give a rundown of the requirements:
A theory starts with a hypothesis (what most people are talking about when they say "I have a theory about..."). A hypothesis is a constructed model that adequately explains a set of observed phenomena. Both Intelligent Design and Natural Selection fit this criteria as hypotheses for the origin and development of species.
In order to "graduate" to a theory, the hypothesis must also fit the requirement of being successfully and repeatably testable. The theory must make some kind of prediction that can be verified. For example, General Relativity predicted that gravity would be able to bend light. Many scientists took his paper as mere philosophizing until
gravitational lensing was observed four years later.
In this sense, Intelligent Design does not qualify. There is no test to see if God made the universe and precisely designed every life form. If you can come up with one, please run it. I'm sure we'd all love to see your data.
Natural Selection
is predictive. When Darwin
proposed that humans derived from a common ancestor with apes, he
suggested that there would be a fossil record that links us all the way back to that common ancestor. While the record is still very much
incomplete, there are many intermediate hominids and hominoids that appear to lead back to a historic link with apes about 6 million years ago. We see the product of natural selection every day in our diseases, as influenza constantly changes to move between species, as bacteria become resistant to our drugs, as moths in England changed color with the industrial revolution.
This is not to say that Intelligent Design is inherently untrue (though there is no reason to believe it is), but rather it does not fit the criteria of scientific theory. As my very devout physics teacher once said "I
know that God spoke to Muhammad, but there is no test to verify miracles. Therefore, miracles are not part of science!"