Perhaps I was wrong about the RSS widget being the culprit.  My feed fouled up again, this time returning the HTML for the front page of my blog.  My suspicions are now aimed at WP SuperCache.  Disabling that seems to have fixed the issue.

Not sure why but I had a few problems with my RSS feed today since upgrading to WordPress 2.7.  On inspection (Feed Validator) it seemed that the RSS feed being returned from my main Atom link was returning the feed of my Pownce messages.  Very strange.  Seems like an odd bug but the quick fix was to delete the RSS widget that displays my Pownce messages in my blog sidebar.  If you’re having similar problems and use the RSS widget you might want to try doing the same.  Go to select widgets from Appearance in the Dashboard side menu and click on any RSS Widgets you have and click Remove.  Be warned I don’t think this can be undone, it suited me as my Pownce feed is pretty much useless now Pownce is closing down.

UPDATE: Don’t forget to clear your cache after doing this.

UPDATE 2: Perhaps it’s the cache and not the RSS widget after all.