This document describes an RSS extension namespace by which podcasters can maintain a single RSS feed but still be able to direct podcatchers to multiple encodings (mp3, m4b) of a podcast and multiple means (HTTP, BitTorrent) by which it can be fetched.
This document also describes how best to use standard RSS feed elements in order to encourage podcatchers to behave how you expect them to.
If you have any questions about this specification, hassle Garth either privately or in public in the ipodder-dev mailing list.
First, you must make XML parsers happy (and let everyone know how to find this document) by adding the podcache namespace definition to your rss tag:
<rss xmlns:podcast="http://ipodder.sourceforge.net/docs/podcast.html"> ... </rss>
iPodder and anything else using a permissive feed parser won't mind if you skip that, but I suspect other podcatchers using strict XML parsers won't be able to parse your feed if you omit it.
These are all currently documented in the Busy Podcaster's Guide to Feed Generation.