Not sure when it happened but I noticed my comments RSS2 feed from my self-hosted blog was not validating. Here’s the trac ticket I wrote for the WordPress bug. WordPress version 2.2.2. If you’re interested in the fix, leave a comment and I can provide more detail if the Trac entry is too vague.
UPDATE: My patched feed-rss2-comments.php file. Caveat emptor (but it works fine on my version 2.2.2 install). Untested on prior versions but it looks like it should work with any 2.x installation.
UPDATE Part 2: My patch was accepted and applied to the WordPress trunk and branch for release with version 2.2.3.