In the Bungie Weekly Update, they introduce the idea of the Halo 3 maketing.
Even though we showed Halo 3 for the first time last May, the official marketing campaign for the game is just now starting to begin. With this kickoff comes the first phase of visual identity – what is creatively being referred to as the “teaser piece.” While it may not be anything new for fans who have been following Halo 3 all along, it’s still very cool, very iconic and evokes a sense of mystery and intrigue. It teases you if you will. Although, it does contain ONE very important piece of information. Cough.
They posted this picture:
On the bottom of the linked image you see this:
So, probably November!
Finish the Fight, this Fall!