All realms will be offline until at least 6:00pm EST / 3:00pm PST. They are having to keep the realms down for this extended period due to errors in the mail system.
Essentially, every piece of mail that was sent prior to the realms going down at 8:00am EST was lost when the servers came back up. Blizzard has said that they are actively looking for a solution to the problem. We're keeping our mailbox post updated with the latest information on that front.
Hopefully this extended downtime is not foreshadowing things to come when Wrath of the Lich King starts to enter circulation tomorrow evening at midnight*.
Update: Please see this post for more information concerning the downtime.
Stay tuned to WoW Insider for more updates.
Are you ready for the challenges that await you in Northrend? WoW Insider has you covered with our 8-day guide to Preparing for Wrath. Also, check out our feature on AddOns new, old and outdated and Essential AddOns for Wrath written especially with the expansion in mind.
Update: It's 6:00pm EST / 3:00pm PST, and the realms are still offline. Blizzard retracted the specific time table, and we're waiting on further updates. It should also be noted that Blizzard's forums are experiencing technical difficulties, so new information might be coming in a tad slow.
Update: 7:27pm EST, realms are still offline. No ETA when they'll be back up.
*Yes, we know. Midnight really is Thursday morning. But if you live on the east cost, you'll be getting it at 9:00pm PST. Which is tomorrow night for a very large portion of the country.