As of today, the game formerly known as Halo 3 : Recon will henceforth be known as Halo 3 : ODST.
The upcoming Bungie project formerly known as "Halo 3 : Recon" will now be known as "Halo 3 : ODST."
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.
Halo 3 : ODST will drop into stores next Fall and the Halo 3 Mythic Map Pack will be coming to Xbox LIVE Marketplace sometime early next year*. (More info as soon as we have it!)
Stay tuned to Bungie.net in the coming weeks and months for development updates and new details on our latest project. In the meantime, please remove "Halo 3 : Recon" from your memory banks and replace it with "Halo 3 : ODST." Also note that all existing content on Bungie.net and Xbox LIVE Marketplace will get replaced to reflect this new name. The announcement trailer is the same one you've already seen or downloaded with just a minor change to the new title card at the end.
Source: http://www.bungie.net/News/Blog.aspx?mode=news#cid16289
personally, i think Halo 3: Recon sounds much cooler, but i guess : ODST makes more sense, since you ARE playing as an ODST soldier..