I asked my TA about this, since she has been on many Disney cruises, including a sold-out Christnmas cruise. She said what kcashner said, that there's still room, that you really can't tell the difference between the sold outs and the not.
I can't wait...my family and I are going to be on the Christmas 2005 Magic. I finally decided on using Costco Travel, and my original price went from $7800 to $7100 for 4 in a cat. 6. This, by far, beat all the other TAs I looked at.