Microsoft has announced the official line on the pre-release Bonus for 
Halo 3: ODST which launches on September 22nd. 
The main point of interest is that Sergeant Avery Junior Johnson is playable as you trace the pre-
Halo 3 story through the eyes of Orbital Drop Shock Trooper. 
The word is that "If you pre-order Halo 3: ODST at participating retailers, you will receive a token to unlock grizzled UNSC hero Sgt. Johnson as a playable character in Firefight."
See the 
full details here.