I like the idea.
We could, for instance, keep track of what part of the show/audio-file you already listened to, even when you played something else last. it's kindoff like a bookmark but an automaticly generated on for podcasts.
We could also have a checkbox list of episodes, the listened ones will have a check.
On sync the "casted" podcasts can be removed and new one added to the list.
If one day there will be a standard format for shownotes with indications of the time of a topic, we could automaticly generate bookmarks of the topics. It could look like a shownote browser.
This are some things we could implement in Rockbox with ease, I'll take a look at it.