As reported by Eloy in the dev chat:
(11:03:24 AM) Eloy: Andrew: Just testing this (books database rss): feed://moodle.org/rss/file.php/297126/1b51bf7f3cab9689af042af1ff4a07f0/mod_data/55/rss.xml
(11:03:36 AM) Eloy: doesn't seem to work
(11:08:25 AM) Andrew Davis: heading home. Ill have a look tonight. Looking at it in the browser (http://moodle.org/rss/file.php/297126/1b51bf7f3cab9689af042af1ff4a07f0/mod_data/55/rss.xml) it says your authentication token is no good.
Can you please have a look at My profile settings -> Security keys and see if your token matches the one in the URL?
(11:09:37 AM) Eloy: it is "guest" user, for public feeds
The problem seems to affect all other front page database activities on moodle.org e.g. Moodle Jobs http://moodle.org/mod/data/view.php?id=7232 has http://moodle.org/rss/file.php/288759/1b51bf7f3cab9689af042af1ff4a07f0/mod_data/54/rss.xml
I'm guessing this problem is general and not specific to moodle.org.