It's four days to Blizzcon, and Boubouille, long-time source of quality information, thinks he knows what the next expansion is going to be called: Wrath of the Lich King. Obviously this supports previous information pointing to a Northrend-themed expansion, as Northrend is home to Arthas, the Lich King himself. The original post on the MMO-Champion forums claiming this to be the expansion name also asserts that there will be a new class: Death Knight! This seems to be less substantiated, but I wouldn't be surprised if it turned out to be true. Rumors aside, we'll almost certainly know for sure in just a few days. Are you excited? I am.
As far as the Death Knight rumor goes, the WoWWiki page lends a bit of support: apparently, Death Knight (as well as Necromancer) were originally planned to be classes in WoW, but didn't make it to release. Furthermore, there exists a green called Triumphant Shield of Shadow Wrath, which is confusing as no shield-using classes (Warrior, Paladin, Shaman) can use Shadow magic. This could just be an oversight in the random-enchant-generating code, but it could also be a hint at a Shadow-wielding, shield-using class...
Update: Over at MMO-Champion, they suggest that this is accurate because the link www.worldofwarcraft.com/wrath/ leads to a "forbidden" error, whereas accessing any other non-existent page on the site will give you a "not found" error. Continue to enjoy your rumormongering, everyone!
Update 2: Again from MMO-Champion, the title World of Warcraft: Wrath of the Lich King was found on the web site of Usk.de, which is apparently the German equivalent of our ESRB (i.e. the agency that assigns content ratings to games). It seems to have a "12" rating; although I don't speak German, I assume that means "suitable for 12 and up."
Update 3: It's official now -- check out all of the news on Wrath of the Lich King.