Ken Whisenhunt confirmed after Titans practice today that Zach Mettenberger will start for the Titans on Sunday. Whiz said "we need to see what he's got" in reference to getting Mettenberger on the field. He said that the decision was made around noon on Tuesday, and that's when all three quarterbacks were informed of the decision.
The only real piece of news here is that Whiz said Jake Locker will be the back-up. They wanted to see Locker throw on Tuesday before deciding if he was healthy enough to be the back-up. They are apparently confident that he is.
This is a new dynamic for Locker. He has never been anything but a professional, so there is no reason to believe this time would be any different.