Bungie announced today it has decided to change to change the name of the game formerly known as Halo 3 : Recon, which is a prologue to the events in Halo 3 that tells the story from an all-new perspective: an ODST (Orbital Drop Shock Troopers) recon marine. As such, the game will now be known as "Halo 3 : ODST." Bungie said:
If you recall, this is our recently announced stand-alone extension to Halo 3 that features an all new campaign played from the perspective of an ODST in the heart of a Covenant occupied New Mombasa. In addition to sleuthing around the city and piecing together the details surrounding your squad's whereabouts, the game will also include some new Halo 3 multiplayer content as well as one or two unrevealed surprises when it's released next Fall.
This new title is a more straightforward account of the experience that awaits and the role you will assume as you explore the streets of New Mombasa. But don't worry, you can still has Recon. (the armor that is, it's still tied to the Vidmaster achievements and will be accessible via the final retail release next Fall).
"ODST" (literally pronounced "Oh-Dee-Ess-Tee" and not "ohdst") is the acronym for "Orbital Drop Shock Trooper." This special forces branch of the UNSC military force is called into action for highly specialized, small scale operations. ODST units were deployed during portions of the Halo 2 and Halo 3 campaign and have also been featured in some of the Halo novels. To learn more about this elite military operation, check out this recent interview with Bungie's Halo fiction guru, Robt McLees.
[SOURCE]