You certainly already heard about the World of Warcraft Facebook Application (official Blizzard FAQ on that here). But it turns out that this was only the beginning of the WoW Facebook integration. Today it was announced that the upcoming WoW auction house for the iPhone app will also be ported to Facebook. And MMO Champion posted a rumor, and they only post those when they have a really reliable leak, that during the Blizzcon on October 22nd a 2D World of Warcraft client for Facebook will be presented. You will be able to play WoW directly in Facebook on an isometric view Flash client!
Of course functionality will be limited to simple questing, no PvP, no dungeons, no raids, you won't even be able to form a group, only solo. And there will be no addons possible, which will come as a shock to some people overly relying on those. Nevertheless this will allow some leveling during your lunch break at work, if your companies firewall isn't blocking Facebook. That is already quite a feat, no wonder it'll still take them a while to actually implement it.
I think a World of Warcraft integration with Facebook is a great idea. The social network features of WoW are definitively lacking, with you being limited to just one guild, and not able to communicate with friends on other servers. Meanwhile Facebook really needs some more engaging games, and street cred with the gamer crowd. Having WoW on Facebook will prove to everyone that Facebook can do a lot more for gaming than just Farmville.