For those of you installing WordPress 1.5 and wondering what in the world is wrong with your RSS feeds, take a look at this: http://mosquito.wordpress.org/view.php?id=903

The work around is to edit the wp-blog-header.php file and comment out lines 112-115:

if ( preg_match('/cgi/', php_sapi_name()) )
@header('Status: 404 Not Found');
else
@header('HTTP/1.x 404 Not Found');

This clears up both the main and comment feeds. This has been marked to be fixed in 1.5.1 and should be available via CVS.