I have to believe that there was some preliminary ground work done prior to this announcement. I cannot imagine Disney making this announcement based solely on a top level "hey I got a great idea" sort of moment. I am sure that there are already drawings that have passed back and forth between the two camps (Disney / Cameron). Otherwise we would not have been give such a specific announcement. They would have just announced that they were working with Cameron on an Avatar project to be announced at a future time. The fact that they announced that it is going to be a land and that it is going to be in Animal Kingdom suggests that there has already been some design work done.
Of course this comes from someone who was just a CAD designer and now works in the power generation industry.... So take it for what its worth. lol
According to Disney by Mark, http://www.disneybymark.com/2011/09/21/news-avatar-at-animal-kingdom/, Imagineering didn't know about this. Joe Rhode included.
Down in the comments, he also mentions that when the Indiana Jones deal was done, Imagineering wasn't involved.