If you want to go there even earlier, take the boat to the Contemporary, walk through the lobby and then its a five minute walk to the MK. I believe the resort boats begin their runs as early as around 6:30 am. We did this on our last trip, which was right in the middle of the Christmas season 2007. We wanted to be at the MK turnstiles as least an hour before opening, and it worked very well to go to the Contemporary.
Hope this helps,
Tromba