I'm not technical enough to know why this "worked" for me, but I figured I'd share it in the event that, somehow, it was the root cause. If someone else is able to completely discount this, please do so!
I was getting the CGI error described here for my RSS feed. I tried the wp-db.php changes...to no avail.
After backing up my database, deleting all the tables, reinstalling WP, and restoring the database...nothing had changed.
Then, I started getting a completely unrelated error when I tried to view the blog itself. I could get to the admin area, but couldn't view plain ol' blog content. That error was much easier to troubleshoot -- index.php was referencing "./wp-blog-header.php," which is actually located in the same folder (the root) as index.php.
I removed the "./" from index.php. Not only did the site start working again, but the RSS feed started working -- no CGI errors!
I can't connect the dots at all. It doesn't seem like these would be related. But, things are at least working for me again.