Halo: Recon was renamed to Halo: ODST because the original title didn't relay the game's "experience," Bungie group product manager Ryan Crosby has told Gameplayer.
“We’ve changed the name because we feel that it is a more accurate reflection of the game experience,” said the exec.
“The ODSTs play a pivotal role in the human-Covenant struggle, and we want to make sure that everyone understands that you play as a member of this elite unit. While Halo 3: ODST is an extension of Halo 3, playing as an ODST makes this experience quite different from others within the Halo universe.”
There's a full interview through there.