Update: A game called Halo Blue just popped up on Wal-Mart's intranet, according to this. Could it be?
Here we go, then. According to this Gamespot "rumour," the Halo game that will be announced in the Microsoft E3 conference tomorrow (yes, of course it will) is a squad-based shooter. No Master Chief included. From the site:
The new Halo game will be a spin-off that resembles "a cross between Ghost Recon and Gears of War." It will reportedly tell a "less cartoony" and "more bloody, violent, and grim" tale of a battle between UNSC forces and the pan-racial religious empire known as the Covenant. The game's atmosphere was described as similar to that of the live-action Halo shorts directed by South African filmmaker Neil Blomkamp (pictured), who was to helm the now-stalled big-screen adaptation of the game.
If that isn't true we'll eat our shorts. We're pretty sure everything's been leaked, now. Tenner for everyone if there's a surprise left.