There is a problem at the Store end at the moment which is preventing people from submitting podcasts: Rob Walch has been in contact with them and says they are working on it, so I should just leave it for a day or so and try again.
Your feed is basically OK. The errors flagged in FeedValidator are not serious but should be addressed.
'itunes:subtitle should not contain HTML: ': The iTunes subtitle and summary tags should be limited to plain text. You've put the code for a non-breaking space in: surprisingly iTunes is actually rendering it but it's not a safe practice in general - it's just a likely to display the code.
'Missing recommended iTunes channel element: itunes:category' - you have no 'itunes:category' tag, and in order to geet a decent dispoay in the Store you should add one - please see this page:
You should restrict the category to those listed at the bottom of that page: don't invent your own as they won't be recognized.
'The recommended <itunes:email> element is missing' - this tag is used to add an email for contact purposes, please see
Note that though it may not be displayed in iTunes anyone can access the feed and read the code, and it will be visible there, so you may want to be cautious. It's not serious if you don't include it.