Apparently the word "recon" is either too prosaic or already featured in too many game titles. Or maybe it just makes too much sense. For whatever reason, the developers of the Halo: Recon campaign for Halo 3 decided a more esoteric name was in order. Hence, Halo: ODST, explained as follows along with instructions for how to literally pronounce it:

"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.

What we have here is yet another instance of someone at developer Bungie believing the strength of Halo is its convoluted storytelling. I'm sure fans will be delighted at being able to finally play as an Oh-Dee-Ess-Tee, but the rest of us will be left wondering whether ODST can be treated with Ritalin.