We've been upgraded twice in three Cruises. First was from two cat 11 GTY to two cat 4 cabins on a 4 day. We boked about six weeks before sailing. Second was from cat 11 GTY to cat 9 on a 3 day. Our most recent 7 day Cruise we booked cat 10 and enjoyed the Cabins very much. Just book what you'll be happy with and enjoy the surprise if you get an upgrade. I am keeping my fingers crossed for 2006 though but if not we loved the cat 10's and are in an area of the ship we want.
When we booked, our agent felt that we had a strong chance of being in Cat 11 or 10s because of there's only a dozen cat 12s. We were hoping for 10's for the extra bathroom space, but a big bonus on going up to 9's!