When submitting to the iTunes podcast directory by following the instructions, I got the following error message:
There is a problem with your feed. Your episodes are hosted on a server which doesn’t support byte-range requests. Enable byte-range requests and try your submission again.
I checked with my host, the excellent ICDHost, and they told me that everything should work for podcasts. I searched around a bit and found one forum post that suggested putting the podcast MP3 files within the WordPress file structure caused the error, becuase of some strange PHP stuff that WordPress does.
So I moved my MP3 files from /www/wp-content/uploads to /www/podcast or thereabouts – outside the WordPress folder structure. I changed the settings in PodPress to ensure the feed knew where to find the files, and then I re-submitted the feed to iTunes. This time it worked, and now the feed is available in the iTunes directory.