The garland in MK is definitely up the week before Thanksgiving. I've gone 2 years in a row that week, 2009 and 2010 and both times it was up!
The Christmas tree, however, is not up until December. I think most of the resorts are getting decorated the week before Thanksgiving too, we stayed in WL in 2009 and asked when the decorations would go up in the beginning of the week, and they told us Wednesday night...sure enough, we woke up Thursday morning, went down to the lobby, and the whole thing was decorated, it's amazing!

I love WDW during the holidays!