I tried to log into Cataclysm repeatedly for 10 minutes, just minutes after release at midnight. It didn't work, the login servers are overloaded. I'm not surprised. I would have been surprised if it had worked, which is why I tried it. Now I'm going back to bed.
[UPDATE: Sleeping helped. The game was up at 6:30 when my alarm clock rang. But alas, now I have to go to work.]