The difference seems to rest on partial playback. New means you haven't listened to even part of the podcast, whereas unplayed means you haven't finished listening to the podcast.
In testing, it seems:
- iTunes for Mac (v. 10.6.3) tips from new to unplayed at 15 seconds in tointo a track
- iTunes for iPhone (iOS v. 5.1.1) tips from new to unplayed at 5% in tointo a track
Note this is based on quick experiments, not cited documentation or the like.