Unfortunately, I don't believe this for a second. Gamer Andy links to a post at Hexus Gaming where one of those "deep throat" style insiders says that Halo 3—which hasn't been announced, shown or even talked about by Bungie—is finished and secretly sitting at the production plants waiting to be mass-produced, packaged and sold. If you believe this, I am moving to Alaska and selling ice. This is a business, and it makes exactly zero business sense to finish a game like Halo 3, press it and release it without a huge tidal wave of hype and marketing. Plus, if the game was done now, there's no way they have even close to enough Xbox 360s floating around in stores to meet demand.

Game Held at Replicators Awaiting the Green Light [Hexus]
Halo 3 So Fast? [Gamer Andy]
Halo Film Script Leaked? 1Up Says 'Maybe'