I would book as soon as possible which means as soon as the dates open up which could mean up to a year in advance. This is especially if you want specific things like a van over a car at the rental agency or a specific room at the hotel. If you don't care what you get, then you can probably wait. Remember how many people will be getting rooms and cars--not just Disney boats--but other cruise lines and land tourists as well. Particularly out of the airport.
I am cruising 7/1/06 and keep check on web sites of the airline, hotel and car agency I use to see if the dates have opened yet. I get pretty specific things, so I want to make sure they are available.
DG