Zarhym posted in response to forum questions that new class combinations, such as Troll Druids, will be available after The Shattering, which will happen shortly before the Cataclysm.
Get your $25 to race change ready!
Q u o t e:
Will the new Race/class combinations (not Worgen/ Goblin), be available prior to Cataclysm? Or do you have to have a Cataclysm account to create an Orc mage or a Troll Druid?
They will be available to everyone shortly before the release of Cataclysm when The Shattering takes place.