If the government "knew for sure" something was going down -- WDW would be closed, or they'd be taking action to stop it. Terrorist threats just don't happen that way. They know for sure that something is being discussed or planned, but not that it's going to happen or cause mass casualties.
I would probably change my plans if there were a credible threat for a specific location during the time period I was planning to be there. Right now, with the threats in Europe I wouldn't change my plans (if I were planning to go there) but if it changed from "somewhere in Europe" to "a mass transportation location in Lisbon" for example, and I had a plan to go there I might change the plan, or fly into another city and rent a car or something.