That is your opinion. There is no way you can prove that IDers assume a designer before they see the evidence. I can say the same for evo's.
It's simple enough to demonstrate really. There's no evidence for the irreducible complexity that ID proponents almost invariably hinge their entire arguments on, so the only thing to fall back on is an assumption that evolution can't do the job as a naturalistic process, which is nothing more than a glorified argument from incredulity.
Then there are comments like Behe's from the Dover trial where he claimed that scientifically speaking, ID was as valid as astrology. Now, does astrology have anything other than an assumption that it works as a foundation? No. Why would Behe make a comparison like that under oath if ID had anything more substantial to back it up? Simple answer: he wouldn't. After Behe managed to admit this sort of information, Dembski refused to testify. We can come up with a few reasons why he wouldn't have wanted to, despite the fact that he had been quoted as saying that if evolution and ID ever went to trial, the scientific theory (i.e. evolution) would be destroyed. However, it essentially boils down to Dembski realising that his evidence wasn't good enough to fool anyone at that trial, especially after such claims as "no evidence for the development of 'irreducibly complex' features" had been shown to be utter lies during Behe's testimony.
To me it sounds like you haven't read much of Dembski's work. Dembski doesn't think all complexity is design, he spends plenty of time detailing "specified complexity". Maybe you missed that part.
I read some of Dembski's work before and after Dover. I found nothing of any real substance to it. Complexity can arise naturally, there's evidence of that, so complexity in and of itself is not compelling enough to warrant the assumption that an intangible designer plays with DNA during the evolutionary process.
The fact that Dembski's work on ID has led to the rest of his department issuing a statement saying that they do not share his views or support his work in the least is also very telling, as most scientific establishments are very supportive of any work that might lead to either discoveries, inventions or explanations. Add to that the issue of Dembski silencing any of his critics in the online community he runs by simply deleting their comments and banning them from the forum indicates that he's not in the least bit interested in discussing his evidence with skeptics, just in glorifying himself to his exiting followers.
The definition of irreducible complexity is solid. If it can be proven in nature, design exists. It doesn't surprise me that many of Behe's examples go unheard and still unanswered. I still laugh at the attempt at explaining the evolution of blood coagulation. However, I'd expect to see such a fierce reaction when a thoery is in crisis.
The definition itself may well be solid, but the examples are utter bunk.
Some further bits of amusement about Dover from wikipedia:
http://en.wikipedia.org/wiki/Irreducible_complexity said:
While testifying at the Kitzmiller v. Dover Area School District trial Behe conceded that there are no peer-reviewed papers supporting his claims that complex molecular systems, like the bacterial flagellum, the blood-clotting cascade, and the immune system, were intelligently designed nor are there any peer-reviewed articles supporting his argument that certain complex molecular structures are "irreducibly complex." [3]
"As expert testimony revealed, the qualification on what is meant by "irreducible complexity" renders it meaningless as a criticism of evolution. (3:40 (Miller)). In fact, the theory of evolution proffers exaptation as a well-recognized, well-documented explanation for how systems with multiple parts could have evolved through natural means."
"By defining irreducible complexity in the way that he has, Professor Behe attempts to exclude the phenomenon of exaptation by definitional fiat, ignoring as he does so abundant evidence which refutes his argument. Notably, the NAS has rejected Professor Behes claim for irreducible complexity..." (Page 75)
Emphasis mine in the next one.
"...on cross-examination, Professor Behe was questioned concerning his 1996 claim that science would never find an evolutionary explanation for the immune system. He was presented with fiftyeight peer-reviewed publications, nine books, and several immunology textbook chapters about the evolution of the immune system; however, he simply insisted that this was still not sufficient evidence of evolution, and that it was not "good enough." (23:19 (Behe))." (Page 78)
All in all, Dover was more than enough of an embarrassment for the ID movement, and by all rights it should have given up its claims at that point.
You mentioned you could say the same for evolutionists, I'd be quite interested in seeing that.