When it's a walking tour, a music lesson, a serial book.
Over on the Futurelab blog, Karl Long has an interesting article on uncommon uses of podcasting which covers these applications and a number of others. I particularly like Karl's semi-throwaway suggestion of an alternate commentary for movies – although why stop there, why not re-voice entire movies?
I also recently read of podcasts being used as pseudo-personal training guides. Handy when you need that little extra motivation to keep you on track – just...eight...more...push-ups...
I've also seen podcasts are also used as meditation aids – over at Zencast, one of the first podcasts they put up was a set of 5/10/20 minute silences which ended with a single bell chime. This solved the "I wonder how long I've been sitting here maybe I'll just look at my watch – is that all?" problem.
There is nothing to say that a podcast must follow the downloadable radio show format – this is like saying all websites should look like magazines. And with the expected growth of video podcasting (sorry, can't use the term vodcasting with a straight face) who knows where the technology will take us?