I've been having the same problem for days. Haven't been able to find a fix. I've validated my feed and successfully added it manually to iTunes. I've tried submitting it to the iTunes directory from different computers, Mac & Windows, from different internet connections, even from different cities - no joy. I contacted iTunes support and they gave me a bunch of useless canned answers.
Without any specific instructions from Apple on configuring your server so that the iTunes Store can reach/read your podcast feed, I have to assume that it's something on their end or bad iTunes code.
I'm wondering if there will be a fix in the next (delayed) version of iTunes, due by the end of the month.