I found this picture on one of the NASA webcams. I wonder if it is a mobile launcher platform (MLP)? If so, is it the one to be used for Discovery? I do not know enough about VAB processing flow, but it might be that the full stack (orbiter, boosters, external tank) be assembled first. When this is done, they might be lowered to the mobile launcher platform. If that’s the case, then my speculation will probably be right. If anybody knows for sure, please drop me a comment. And, of course, I’ll also try to find out…