I don't get the impression anyone other that Oswalt and Pettitte are locks. Backe, Rodriguez, Buchholtz, and Nieve have all been inconsistant in ST. Backe has the most experience out of the 4, so it stands to reason he'll probably get a longer opportunity to turn it around from ST, if ST is even pertinent to regular season. But Wandy, Buchholtz, and Nieve all have to show more positives than negatives because there is a list of guys in AAA ready to push them: Hernandez (who may still get the #5 spot in the rotation), Hirsh, and Astacio to name a few. Barthmaier could also be an option for pitching help.
The backend of the rotation is young and inexperienced but they have a number of candidates they can trot out there should anyone falter.