Have the same problem after upgrading to wordpress 2.7. Not working (change myblog for https://www.proyectalis.com if you wish to test yourself):
https://myblog/feed/
https://myblog/feed/rss/
https://myblog/feed/rss2/
https://myblog/wp-rss.php (this is a redirection to /feed/rss/)
https://myblog/wp-rss2.php (redirectio to /feed/rss2)
They give a “Myblog: page not found” (404).
Working:
https://myblog/feed/atom/
https://myblog/comments/feed/
Checked no spaces left after ?> in wp-config.php (only file updated). Disabled ALL plugins. Re-saved permalinks settings. Checked for strange characters in category names (accents and so on, this COULD be an issue, as there are strange characters in the database, only they are not showing up on the site as I specified “latin1” in wp-config). Nothing.
LOTS of tickets and bugs open with the same issue (try searching for “wordpress feed 404”). No good answer yet. This should be a priority issue, as loosing your feed is a big problem and many people could happen to just have not noticed their feed is not there now after upgrading!!