It works a little like this.
MIchael Succo, who goes by Captain Corduroy on the Something Awful forums, occasionally shows up there to spread rumors and stir the pot. It's not too surprising, as half the Internet has a foot in that door.
Last week, he does his thing, leaking a bunch of information about the forthcoming World of Warcraft expansion. Blizzard filed for a trademark on Catalclysm earlier this year, and that's turned out to be the new x-pack's name. A lot of similar information also showed up on MMO-Champion.com, which was either a completely separate leak or an MMO-Champion blogger striking while the iron was hot.
Fan speculation's been for a while that the next expansion would either pick up on the ongoing druidic plotline involving the slow corruption of the Emerald Dream, or would involve a journey into the Maelstrom, the swirling vortex in the center of the Azeroth world map. There's an old "design document" that gets posted and reposted a lot that indicated the next x-pack would involve the islands in the middle of the ocean, like Kul Tiras, Undermine, and Tel'Abim.
That isn't actually the case. Cataclysm was teased to be, instead of an expansion pack, a complete reinvention of Azeroth itself,. Two new playable races have been introduced, with the goblins officially joining the Horde and the wolflike worgen coming out from behind the Greymane Wall in Silverpine Forest. The villain of the piece is longtime Warcraft nemesis Deathwing, whose influence can be felt all over every expansion to date, although somewhat indirectly in Burning Crusade. (He had an agent in place dealing with the orcs that had enslaved the netherdrakes in Shadowmoon Valley.)
At Blizzcon yesterday, the leaked information turned out to be absolutely true in every regard. The new expansion blows up Azeroth and starts it over again, destroying big chunks of the landscape and redesigning it entirely. The Barrens gets split into two zones, Azshara is now a lowbie Horde zone, the Burning Steppes have blown up again, Thousand Needles is full of lava, and Uldum in southern Tanaris will be a leveling zone for 82-84-level players.
The odd thing about Cataclysm's features list is that it reads like fans wrote it. I actually had to apologize to somebody in my guild who's been vocally asking for dwarf shamans for months now; other odd changes include the ability to roll tauren paladins, gnome priests, human and undead hunters, blood elf warriors, and more, as well as - and this is probably my favorite bit - heroic Deadmines.