I recently noticed a feature on my 2nd gen iPod nano and have been experimenting with new ways to use it.
When you navigate via main menu podcasts, the display shows a bullet icon next to those podcasts that have not yet been listened to. However, when you navigate to podcasts via the genre menu, this "not yet listened to" icon is not displayed. After some experimenting I think I figured out what is going on. The iPod puts items in the main menu podcast list when Media Sub Type=Podcast; whereas podcasts viewed via the genre menu rely on Genre=Podcast.
I think this "not yet listened to" indicator can be useful for multi-chapter audio books when you want to keep track of which chapters you have already listened to. To enable this you need to manually set Media Sub Type=Podcast for multi-chapter audio books (leaving Genre=Book or whatever you use for audiobooks).
An added bonus of doing this is that bookmarking will now be enabled for the audiobook.
This trick does have an inconvenience. The iPod assumes all podcasts have only one chapter. This means that if you play an audiobook chapter via the podcast main menu, then it will play the single file and stop, rather than automatically resuming with the next chapter. Therefore, the best way to use this feature is to navigate via main menu podcast to find the last chapter you listened to, then back out, and navigate to the chapter via the genre (or artist or album) menu and press play. The iPod will then play the remaining chapters in sequence as expected.
If anyone has any deeper insight into what is going on here, or better ways to take advantage of this feature, I would appreciate it if you would share your thoughts.
As an aside, I did some experimenting with setting Media Sub Type=Audiobook. Nothing useful seems to result from this. Items do show up in the audiobook main menu however no "not yet listened to" icon is displayed, and like for podcasts, the iPod seems to think all audiobooks (as defined by Media Sub Type=Audiobook) have only one chapter.