Master Chief is in Halo Reach
As you can see in the picture above. Master Chief, Cortana and Sgt Johnson plus ODST Buck are in the game.
Recent Bungie post clearly shows Master Chief and Cortana as options in the Halo: Reach Armory. Note these are just the initial pieces; unlocking higher levels through playtime will grant you access to new components. Interesting that Master Chief, Cortana and Sgt. Johnson are present. Multiplayer skins, perhaps?
Screenshot of Armory:
Halo Reach due release on September 14th.