Toosten Flight mode (free) does this:
http://pocketgear.com/software_detail.asp?id=17495
You'll have to do the shortcuts manually though. You can probably tie into another free app like CT Scheduler:
http://smartphone.net/software_detail.asp?id=3072
CT Scheduler is technically for smartphones but it should run on PPC just fine. So you just link the two togther --> CT Scheduler runs the Flight Mode shortcut at time X.
There is also AutoFlight for a small price:
http://www.mobihand.com/treocentral/...t.asp?id=11813