So it is possible to do Podcasting with a typepad account. Not with the standard Typepad tools so far, unfortunately.
Feedburner is a service that can transform your RSS feed from typepad (and other sources) in several ways but for Podcasting the most important feature is that it can add an enclosure tag.
You tell feedburner to create an enclosure by adding rel="enclosure" to the tag that links to the file you want to enclose. In my test post, this looks as follows:
<a href="/PodCasts/FeedburnerTest-041129.mp3" rel="enclosure">FeedburnerTest-041129.mp3</a>
The result in the Feedburner feed is:
<enclosure url="/PodCasts/FeedburnerTest-041129.mp3" length="189405" type="audio/mpeg" />
It looks like Feedburner figured out the length (size in bytes) of the file and determined that the file type (MIME-type) is audio/mpeg.