I don't think that our failure to find WMD's is going to change any opinions about the decision to invade Iraq. Those who supported the decision will continue to support it and those who opposed it will continue to oppose. Few people are open to the suggestion that we knew that there weren't any WMD's. And, who knows, we might still find some WMD's.
I think the biggest effect of all this will be felt the next time we try to gather international support for action based on facts that are doubtful. At least for awhile, people are going to be harder to persuade based solely on our "intelligence sources."