Maintenance has been extended until "approximately" 2:59pm EST. In the official post on the forums, Blizzard recommends looking into your account security and making sure that everything is up to date. I agree and to that end, I'll also link you to the WoW Rookie account security roundup.
We'll update this post with any changes to the realm status.
Update 8:14pm: All realms are up, theoretically. However, many are having severe latency issues and others can't get past the character loading screen. Blizzard says they are working on it.
Update 7:21pm: All realms are up, but expect some latency issues.
Update 6:24pm: Some realms are up. They will give us another update on the rest of the realms by 7pm EST. Bored? Come visit It came from the Blog, Zangarmarsh U.S. Horde side. Send a tell to Robiness or any Lurker for a guild invite.
Update 5:10pm: Even more issues have been found and they'll get back to us at 6pm EST.
Update 4:21pm: They are resolving more issues and have extended maintenance again to 5pm EST.
Update 3:26pm: Maintenance has been extended to 4pm EST.