After the first maintenance extension, many suspected the downtime was related to the Battle.net account collation that Blizzard is implementing with Wrath. The servers came up briefly, but then the word went out that there was a problem with the in-game email system. Blizzard, trying to assure the community, sent a warning email to players letting them know that there was a problem with the in-game email system. The problem is that they notified everyone by emailing players through the messed up in-game mail system. You know, the one on the servers that were going back down to fix the mail problem. To their credit, they did post the message out on the forums, but there was a small catch there; the forums were up and down all day.
Update: As of 1pm EST, the WoW login screen states that all servers are back up and the service should be working as usual.