Hahahaha, fooled you!
Yes, there is a game called Halo 3: ODST in the works. No, it is not a new development.
Bungie has announced via their weekly news post that what you once knew as Halo 3: Recon is now the more straightforward Halo 3: ODST.
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).
*Whew!* For a second there, it looked like the Fall 2009 season was going to contend with Fall 2008 for the title of Most Inconsiderate Games Release Season Ever...