Intelligent Design (ID) can be thought of as having two parts: a negative part that claims that evolutionary explanations of various biological features of the world are unsatisfactory, and a positive part that says that these features are best explained by positing intelligent agency.
Is Intelligent Design a scientific theory? Not really, if only for the simple reason that the positive side has not been worked out with a sufficient level of detail to merit the term "scientific theory". If a corpse is found with a certain set of wounds, and scientific examination makes it very unlikely that the wounds were inflicted by non-agential processes because the wounds spell out a word, the conclusion "An agent did this" is a fine one for a forensic scientist to draw. But this conclusion, while scientific, does not seem to merit the term "scientific theory". Nor is the issue that this is an isolated case. If lots of corpses with such wounds were found, the claim that each of them is the result of intelligent agency is still not a scientific theory. I think one important reason for this is that there is a serious lack of detail here. Likewise, it would not count as a scientific theory to say that the deaths were the result of "natural causes", with no further specification of the cause. (The lack of detail is related to the accusation of unfalsifiability; obviously, the less detail is given, the harder it is to falsify a view.)
Now, individual proponents of ID might give more detail than the mere claim that agency is behind the biological processes. Thus, they may specify how many agents were involved (e.g., one), where the agents intervened (e.g., at boundaries between species, or maybe of some higher taxa) and how they intervened (e.g., by miraculously causing mutations). Once more detail is given, they have more hope that the claim will become a scientific theory.
But even if individual proponents of ID give more detail, it will still not be correct to say that ID is a scientific theory. Rather, ID will at best be a family of disparate scientific theories. Merely rejecting evolution and holding to agency is not sufficiently contentful to unify the family into a single theory, just as George who thinks the butler did it, Patricia who thinks aliens did it, and Hercule who thinks it was suicide do not hold a single theory, even though they all agree that the death was the result of agential design rather than an accident.
This is important vis-à-vis one political consideration. Some folks would like to have ID taught in school as a theory alternative to evolution (interestingly, I have been told that the Discovery Institute does not take this position). But if ID is not actually a single scientific theory, then it is not parallel to evolution. For neo-Darwinian evolution is much more of a unified theory, although of course individual evolutionary scientists hold to variants of it. Now, one particular positive theory falling under the ID might perhaps be an alternative (whether good or bad) to evolutionary theory. But no one particular positive ID theory has sufficient acceptance even in the ID community as far as I know.
At the same time, the claim that ID is not a scientific theory is compatible with ID being science, just as a particular conclusion of a forensic scientist may not have sufficient detail to count as a scientific theory, but may nonetheless be a scientific conclusion. For, science is more than just the production of scientific theories. (For instance, the criticism of scientific theories is also a scientific practice.)