Microsoft has seemingly announced, according to Wired, that the Warthog that'll be used in Halo 4 will feature in Forza Motorsport 4. The Warthog isn't driveable in the game, but is viewable via Autovista. Cortana will tell you the background history of the the vehicle and its "prowless," apparently. Those attending HaloFest at PAX this weekend will be able to see it in action with Forza 4's Autovista.
Halo 4 launches next holiday. Forza 4 launches this October.