All three rotations are available at both main and late seatings. So basically there are 2 seatings in each dining room per night, or 6 total seating possibilities per cruise (2 seatings, 3 rotations).
If there's a specific rotation you wish, you can request it but it is not guaranteed. According to our Head Server on our first leg of our June 2010 B2B, they don't make up the seating assignments until a day or two before embarkation so there's no way to know what you're assigned until they give you your key to the world card. You can always go to the dining change location on embarkation and see if they can accomodate any changes if you wish one once you check in.