Science is the activity of an organized group of people. This group of people have governing bodies. These bodies have determined that ID is not "science". Moreover, federal courts as well have officially determined (until it is overturned) that ID is a version of creation, and not "science."
Of course the ID movement disagrees, and dispute this. Even as they dispute this, ID has to acknowledge that they are not part of mainstream science, as it currently stand. Moreover, it is clear to most people (even ID people) that ID uses different "rules" in their version of science than we do in mainstream science (case in point the discussions with @Cornelius_Hunter).
So it is usually self evident to say that "ID is not part of mainstream science." I prefer saying it this way because it avoids calling ID "pseudoscience," which is an unnecessarily inflammatory term (whether or not it is accurate).
Some of the key statements that unambiguously show what the federal courts and scientific bodies think about ID are here. I think this is required reading for anyone who cares about the OP's question. One does not have to agree with the decisions of these bodies to recognize that these are the decisions that have been made. Until these bodies reverse their position (and so do the federal courts), ID is not part of mainstream science.
And many many more: https://en.wikipedia.org/wiki/List_of_scientific_bodies_explicitly_rejecting_intelligent_design
And of course, the Dover Decision: https://en.wikipedia.org/wiki/Kitzmiller_v._Dover_Area_School_District#Decision
The overwhelming evidence at trial established that ID is a religious view, a mere re-labeling of creationism, and not a scientific theory. (page 43)
After a searching review of the record and applicable caselaw, we find that while ID arguments may be true, a proposition on which the Court takes no position, ID is not science. We find that ID fails on three different levels, any one of which is sufficient to preclude a determination that ID is science. They are: (1) ID violates the centuries-old ground rules of science by invoking and permitting supernatural causation; (2) the argument of irreducible complexity, central to ID, employs the same flawed and illogical contrived dualism that doomed creation science in the 1980s; and (3) ID's negative attacks on evolution have been refuted by the scientific community. … It is additionally important to note that ID has failed to gain acceptance in the scientific community, it has not generated peer-reviewed publications, nor has it been the subject of testing and research. Expert testimony reveals that since the scientific revolution of the 16th and 17th centuries, science has been limited to the search for natural causes to explain natural phenomena. (page 64) [for "contrived dualism", see false dilemma.]
ID's backers have sought to avoid the scientific scrutiny which we have now determined that it cannot withstand by advocating that the controversy, but not ID itself, should be taught in science class. This tactic is at best disingenuous, and at worst a canard. The goal of the IDM is not to encourage critical thought, but to foment a revolution which would supplant evolutionary theory with ID. (page 89)
To be clear, the ID movement strenuously objects to this ruling and these statements. See Casey Luskin's (a JD) work on this here: http://www.discovery.org/f/1372. However, this is currently and effectively the "law of the land" now. Until this decision is reversed, and ID is not explicitly rejected by almost every major scientific body, ID is certainly not mainstream science.
They can continue the work they care about, just as creation science advocates at the Creation Research Institute have for decades. Their chances of finding their way into public science curriculums are about as likely as AIG and YEC getting in.