How to get iOS 6 podcast app to resume the last played podcast
If you run enough other apps for Podcasts to be unloaded from memory, resuming from the lock screen will start playing the Music app.  There's no way around it.  The only thing you can do for now is to delete the Podcasts app so you can play from Music again (after quitting and restarting it), and join the thousands of other people giving it 1 star in iTunes.