The fabled Halo 3 "expansion" has now been announced as Halo 3 Recon, a complete standalone prologue campaign featuring ODST Marines. So far, no details have been provided, other than that the game will be available for retail in Fall 2009.
As Nick "bapenguin" mentioned in the latest Co-opticast, Bungie has a special place in their heart for cooperative gameplay. They do not disappoint with confirmed 4 player co-op for the upcoming title.
Consider the Halo 3 racing game ("Halo Kart") rumor debunked, and we'll be sure to splash a new post when the official fact sheet comes down.
In the meantime, check out the official intro here or see the awesome NEW trailer after the "Read More" link.
Source: Bungie.net