In a very short answer, the devs confirmed in the recent Twitter developer chat that the new race and class combinations will not go live until Cataclysm itself goes live. In a way, this isn't too much a surprise. Despite some rumors to the contrary, we didn't get death knights until the first day of Wrath, and we had to wait for Burning Crusade to play draenei and blood elves. So with that in mind, it makes sense that we have to wait until the last possible moment for yet another new breed of player characters.
That said, it is probably true that the new race/class combinations are a bit different from the examples above. After all, they simply take existing races and existing classes and put them together, so it might seem that it wouldn't be too hard to just turn them on.