World of Warcraft - Cataclysm *confirmed*

Q: Will I need the expansion to be able to experience the Cataclysm changes?
A: When the Cataclysm occurs, it will occur for all players, whether they have purchased the expansion or not--you will no longer be to play in the original version of Kalimdor or the Eastern Kingdoms. However, certain features such as the new zones, new races, and new level cap will only be accessible to players who purchase the expansion.