Thanks for this thread - I'm seeing the same issue after upgrading from 2.8.1 to 2.8.4.
My feed has been functioning with no problems for a few years through WP and FeedBurner. But a couple weeks ago I upgraded to WP 2.8.4 (from 2.8.1) and now although I have two new posts on my home page (http://www.softwarebyrob.com) they are not appearing in my feed (http://feeds.feedburner.com/SoftwareByRob).
I have re-synched the feed, I have disabled the FeedSmith plugin, and I have disabled all of my plugins but nothing has any effect. Note: since then I have re-enabled them.
I'm also confused by the fact that this URL should be my RSS feed source: http://www.softwarebyrob.com/rss2, but even when I disable the FeedSmith plugin, when I hit http://www.softwarebyrob.com/rss2 it still takes me to FeedBurner. I can't figure out any way to keep FeedBurner from redirecting my feed (I've even gone and looked in .htaccess and there's nothing fishy in there).
At this point, after 3 hours of troubleshooting, I am throwing up my hands. Any help is greatly appreciated.
BTW - I just upgraded to WP 2.8.5 a few minutes ago and that doesn't help. I also confirmed I have the latest version of the FeedSmith plugin.