I think the passive part of this could be really interesting - starting with a simple "tag cloud" of keywords by frequency linking to audio snippets that mention them, it'd provide a way to index conversations during the day for future reference (or processing).
* conversation length
* participants
* location
So you could search for, say, that long conversation I had in the park with Bob.I'm not sure how easy it is to identify/track different participants in a conversation.
Edit:formatting