We recently switched our podcast over to using Feedburner as a proxy to our original RSS feed. Shortly thereafter, iTunes started showing 0 episodes.
Here's our original RSS feed:
Here's our Feedburnerized feed:
Here's the podcast page on Apple's site:
The enclosures are still there, with the appropriate type. Feedburner has changed the length of the enclosure from the empty string to "0", which after checking the spec is technically correct.