I am not using channels, and I have this same problem. I turned on channels just to see if the default Podcast channel would have it one, but it didn't. I turned channels back off.
It's been fine for over a year (I'm on episode 50), and this just started, about the same time as eznat777 (August 10th for me). It was happening with episode 1 only, which I have set as the Feature Episode, to show at the top of iTunes. I turned off the Feature Episode checkbox in the main Basic Settings tab, and the feed then validated. Looks like it may be a bug with that option.
I resync'ed my feed with Feedburner and then it, too, was valid. iTunes worked immediately after that (it showed none of my episodes, previously).
FYI.