Leland, single.php is called when a single post is being displayed, multiple.php for multiple posts. Just to be clear, while Habari will search for podcast.single.php, your theme doesn't need it in order to display your podcasts. See http://wiki.habariproject.org/en/Template_File_... for how Habari resolves which template to use.
I haven't seen what WordPress is doing since 2.3, I think, but I think Habari will compete in this area pretty well, actually. We'll see over the next couple of releases.