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.