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 at 15 seconds in to a track
- iTunes for iPhone (iOS v. 5.1.1) tips at 5% in to a track
Note this is based on quick experiments, not cited documentation or the like.