Lucas' podsinking post, pointing to Danny's eyeProdding post, prompted my posting of a little Perl script I've been playing with for a couple of days. Download it here if you want to test/try it out.
The preferences file contains a list of MP3 blogs, which are passed through Playr to generate M3U playlists. The MP3 files are then downloaded with curl (via Coral, if it's working fast enough), stored in appropriate directories and added to corresponding playlists in iTunes.
Run daily with cron, you'll have constantly updating MP3 blog playlists that you can listen to offline.
Update: M3Ucast now displays notifications when new songs are added, via Growl.
From this you can deduce that RSS enclosures could be unnecessary for broadcasting (nichecasting?) audio files, unless you *really* only want people to have to subscribe to one feed for all your content and *really* don't want to provide a fulltext RSS/Atom feed (otherwise the program could just parse the posted media link out of the text instead of needing to deal with enclosures).
M3U files (or plain HTML files or folders if used via Playr) suffice for audioblogging, with the added benefit that they can be bookmarked directly in media players and used for streaming rather than downloading, if the user prefers.
Update: Having seen how elegantly NetNewsWire now handles enclosures, I can see that they can be useful in certain circumstances.