Well, we've also had a much more vocal minority arguing for the inclusion of ID in science class, which leads to a much more vocal opposition to it. The Dover Trial perhaps acting as the clash between the two, with science and the side of right coming out on top.
It may be that more people have seen through the sheep's clothing, or it may be that more people are worried about being "outed" as an ID supporter after the trial. One of the problem's with polls is that people can be swayed by wording as well as their own psychology. There is no guarantee anyone's answering truthfully.