We were at VWL on Nov. 14 '04 and it was already decorated for Christmas, tree up in the WL lobby as well. We were just there one night, after DCL cruise, don't know about other resorts.
I think there is a website that has a calendar of the decoration set up. I'm so sorry that I can't remember more of the details. Hopefully someone else has some info.
We were at BCV Nov 7, 2004 and the resort decorations were already up at BCV and Boardwalk as well. Not in the parks yet, though they were starting to put them up.
Xmas decorations take weeks to months to put up. Most resorts will have them installed by the weekend after turkey day, and the parks by the first of December -- although some stores in the parks will get them earlier depending on how installation is going.