We were in the MK on Friday (11/9) and many of the decorations were up and (believe it or not) some of the background music had switched over to holiday themes. First Very Merry Christmas Party is Monday (11/12), so I expect that most decorations will be up by morning.
Most of the decorations are already up. The gingerbread house is in full swing in the Grand Floridian, the trees are up in Epcot & MGM, Main Street was decorated. Even LeCellier was decorated. We arrived on 11/4 and left on 11/10 so at the beginning of the week we saw the Halloween decorations and by the end of the week it was fully decorated for Christmas!