We were there thru 1/5/06. The majority of the decorations were still up when we left. Along about the 3rd the outdoor trees at the Contemporary were being taken down and the huge wreath at our resort was down that night, but by and large most stuff including the piped in holiday music was still up when we left on the 5th.
The O'sbourne lights are up til 1/7/07 also.
As for purchasing Christmas Decor, there are a number of year round shops in the parks and DTD.
Happy planning..