Linda: It goes like this: We can, when it really comes down to it, never really be 100% sure of anything. Basically, we can only say that there are no white crows because we have so far not seen one (that I know of, at least). But that doesn't mean one exists somewhere. It may simply not have been discovered yet. So, the scientific mindset may be open for one existing. Of course, the more crows we count, and the more of them that are black/dark grey, the more confident we can be that there aren't any white crows.
So, we're basically "proving" a negative mostly by giving evidence of a mutually exclusive positive. As long as the evidence supports the positive, we can assume it'll keep on doing so. Of course, as soon as some other evidence (which is also a positive, except it's a positive for a different direction) is brought up, one has to regard this.
So, technically, life on earth may still have been intelligently designed. Just that whoever did the designing (and presumably initial creating) hid the tracks so well we can't see them.
BUT, it's not our job to say that there is no evidence of a designer, therefore it doesn't exist. It is the job of the IDers to come with some actual, positive evidence that their Judeo-Christian god (which ID is really about 99% of the time, no matter how many times people like Behe lies about this) does exist and designed eyes, intelligence, etc.
So far, they haven't shown anything but pure speculation, and a lot of claims that have in fact been wrong. Mostly these claims are on the line of "The eye could not have evolved, because what use is half an eye?", which is a typical argument from incredulity. And arguments from incredulity don't cut it in science.
ETA: Oh, and if the plethora of answers seem a bit confusing, I'd personally go with HappyCat's post. I think s/he best explains what the ID creationists are really doing.