From a blue post:
You should be able to purchase [the expansion] at midnight Eastern (at participating retailers) and then would be able to play at midnight Pacific Standard Time. (3a.m. Eastern)
Some east coasterner’s are gonna be PISSED that they stood in line in the cold for a game they coulnd’t play for 3 hours. Maybe Blizz rigged the install to take 3 hours.
Watch out for incoming QQ.