This is the false dichotomy that the ID movement is trying to transcend. Evolutionary theory may correspond with factual evidence or not. ID theory may correspond with factual evidence or not. All IDers are asking is for a judgment regarding which theory best explains the available data.
Well, that's the rub though. In many cases, IDers are coming to the table with nothing except demands that they be listened to. By that standard, pretty much anybody should be able to demand a hearing from "science."
Those on the "science" side have a variety of reasons for not wanting to discuss "design" in the first place. The lack of some objective basis on which to discuss ID simply makes it easier for them to dismiss the concept out of hand. (We cannot dismiss, btw, the fact that some on the "science" side are ideological, rather than scientific, in their rejection of the idea.)
The second step in gaining "respectability" would be for the ID contingent to develop a set of "design markers" -- properties that can (to some extent) distinguish between "designed things" and naturalistic phenomena. The SETI folks face a very similar problem -- it might be helpful to the ID community to see how they're approaching it.
The first step, though, is I think still not well-defined: what is it that ID proponents hope to gain from this debate? I think there are all kinds of competing goals, and some of them have nothing at all to do with science. It seems to me that if the ID proponents are going to challenge science on the matter of life, then those goals first need to be identified and understood.
The first, and most important, question is this: why do IDers feel it necessary to conduct a debate with scientists?
It'd be interesting to see how some of the folks on this thread will answer the question.