In a post on the official Blizzard forums today, Community Manager Bashiok confirmed the existence of eight total raid bosses in patch 4.2
seven in the Firelands, and one new boss in Baradin Hold, all with unique models, animations, and effects. (No word yet on how many will have stubby little feet.) Further, it has been confirmed that Firelands, in Bashiok's own words, "is the
raid in 4.2," and will be available for testing soon as part of the "aggressive development cycle":
Yes, Firelands is the raid in 4.2, and 7 is the total number of bosses.
We think 7 is our current ideal for number of bosses in a raid. (We launched Cataclysm with two full raids of 4-6 bosses each.) We're also spending a lot of time making the Firelands bosses as awesome as possible - - creating unique models, animations, effects, sounds, etc. etc. Previously a lot of bosses were larger versions of existing models, which was fine, but that tradeoff is made somewhere. We shift to making fewer but more epic boss fights and there's a tradeoff somewhere. We're concentrating our efforts into a smaller number of fights so that each fight is bigger and better, they're still obviously going to be extremely challenging, but once it is on farm you're not having to spend two, three, maybe four nights just to clear it because the raid is so huge.