Once sharding is allowed in Classic..

See info here:

Q: ARE THEY SHARDING?! I WONT PLAY IF THEYRE SHARDING!!
A: It has been floated by the dev team as one way to help combat the expected overcrowding at launch of lowest level starting zones. BUT IT IS NOT YET GUARANTEED TO BE IN THE GAME. Naturally the forums have been a source of calm, rational discussion on the matter.