So, I have two RSS feeds on my site (http://www.sjbmx.com) that I routinely use: http://www.sjbmx.com/feed and http://www.sjbmx.com/comments/feed. Both have been down for about a week, since before I upgraded to 2.9.
I've tried opening the feeds in the latest builds of Firefox and IE to no avail, and sites that read my feeds - with the exception of Google Reader for some reason - also run into trouble reading the feeds.
This lead me to believe that the feeds were broken, and Googling that told me to reinstall clean versions of all of the various files involving RSS feeds. I also reinstalled all of the various files in the template I'm using. However, this has not fixed the problem.
I've just now done a manual reinstall of every WordPress 2.9 file, and the issue's still there...