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.
Late last night the majority of the issues seemed to have been resolved, and Blizzard cautiously flipped the switch, letting players back on the servers. Only there was a problem with the PvP gear vendors, who seemed to think that it was a good idea to give out several pieces of their inventory for free, and without any type of PvP requirements. While many of the players thought this was good fun and snapped up their (knowingly-temporary) rings, trinkets and other odds-and-ends, Blizzard realized the mistake and yanked the servers offline. As of this 10:15 EST this morning, the servers remain offline, with no firm ETR.
How today's continued server outage plays out for World of Warcraft's player-base is anyone's guess at this point. That said, we're really glad we're not the Blizzard employees slated to hang out with the crowds at various launch parties this evening.