I have recent experience and can clarify a bit further. You can book up to 2 staterooms for each household currently represented on the cruise for a single future cruise at the full discount of 10% off, reduced deposit of 10%, and on-board credit. Any additional cruises booked for other dates or additional rooms for anyone will be at the reduced discount of 10% deposit and on-board credit. Note that the person on the cruise has to pay the deposit with their own credit card.
For example, my family of 4 (me, DH, 2 DS's) was in 2 cabins on our last cruise, and I booked for my Mom and I for March 2013 at the full discount. I booked 2 cabins for me, and 1 for my Mom for Nov 2013 at the reduced discount. Assuming prices haven't gone up by the time we're cruising in March 2013, while my Mom and I are in a single cabin, we'll be able to book my 2 cabins, and her 1 cabin for Nov 2013 at the full discount because we live in separate households.