I can't find the source for this on Wired's own site, so presumably it's only in the print mag for now. Original source found (thanks, Zuffox). Mike Morhaime dropped the following uber-hint whilst chatting to the US tech mag:
"Let's just say it's going to be different and it's not going to be a sequel to World of Warcraft. It will be different. We're not trying to replace World of Warcraft with this new MMO. We're trying to create a different massively multiplayer experience, and hopefully World of Warcraft will still be going strong when that one is released."
Hmm. I'm still banking on it being something Eve-y or Planetside-y, possibly in the Starcraft universe. It seems hugely unlikely Blizz will move to a new IP - but it would be nice. Do spill your own theory-o-thoughts below.