Do I need to own Cataclysm to make a character using one of the new race/class combinations like troll druids or dwarf mages?
If not, are these combinations available now, or do I need to wait until Cataclysm actually launches on Dec. 7th?
Like the content within the most recent 4.0.3 patch (i.e., the shattering), all the changes and new quests, etc. are available to anyone with the base game.
According to the official patch notes: http://us.battle.net/wow/en/blog/1214741
The new race / class combinations are already available in the game. (See the 2nd bulletpoint, reprinted below):