Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 6334)
+++ wp-includes/functions.php	(working copy)
@@ -861,7 +861,7 @@
 	$feed = preg_replace( '/^_+/', '', $feed );
 
 	if ( $feed == '' || $feed == 'feed' )
-		$feed = 'rss2';
+		$feed = get_option('feed_format'); // default is rss2
 
 	$hook = 'do_feed_' . $feed;
 	do_action( $hook, $wp_query->is_comment_feed );
@@ -1484,4 +1484,4 @@
 	return abs( intval( $maybeint ) );
 }
 
-?>
\ No newline at end of file
+?>
Index: wp-admin/includes/schema.php
===================================================================
--- wp-admin/includes/schema.php	(revision 6334)
+++ wp-admin/includes/schema.php	(working copy)
@@ -234,6 +234,9 @@
 	// 2.2
 	add_option('tag_base');
 
+	// 2.4
+	add_option('feed_format', 'rss2');
+
 	// Delete unused options
 	$unusedoptions = array ('blodotgsping_url', 'bodyterminator', 'emailtestonly', 'phoneemail_separator', 'smilies_directory', 'subjectprefix', 'use_bbcode', 'use_blodotgsping', 'use_phoneemail', 'use_quicktags', 'use_weblogsping', 'weblogs_cache_file', 'use_preview', 'use_htmltrans', 'smilies_directory', 'fileupload_allowedusers', 'use_phoneemail', 'default_post_status', 'default_post_category', 'archive_mode', 'time_difference', 'links_minadminlevel', 'links_use_adminlevels', 'links_rating_type', 'links_rating_char', 'links_rating_ignore_zero', 'links_rating_single_image', 'links_rating_image0', 'links_rating_image1', 'links_rating_image2', 'links_rating_image3', 'links_rating_image4', 'links_rating_image5', 'links_rating_image6', 'links_rating_image7', 'links_rating_image8', 'links_rating_image9', 'weblogs_cacheminutes', 'comment_allowed_tags', 'search_engine_friendly_urls', 'default_geourl_lat', 'default_geourl_lon', 'use_default_geourl', 'weblogs_xml_url', 'new_users_can_blog', '_wpnonce', '_wp_http_referer', 'Update', 'action', 'rich_editing');
 	foreach ($unusedoptions as $option) :