Bungie announced that the new “Halo 3: Recon” will now be known as “Halo: ODST.” They don’t give a reason why but explain the meaning of the letters:

“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.