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.
But at the same time, Blizzard is focused on upgrading and integrating the new player experience with Cataclysm, which means revamping or adjusting starter zones and streamlining quests and mechanics for low levels. With that in mind, what better way to kick the whole thing off than with a new set of playable characters at the start of Cataclysm? They're also obviously tying in at least some of the new class/race combinations with Cataclysm lore, such as night elf mages and tauren paladins.
Whatever the reason, you'll definitely have to wait a while longer to level that dwarven shaman you've always wanted, and you may be a few weeks late to the end game because of it. Then again, if we're lucky, it won't take them too long to unlock race changing for the new combos.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.