Index: wp-includes/feed-atom-comments.php
===================================================================
--- wp-includes/feed-atom-comments.php	(revision 0)
+++ wp-includes/feed-atom-comments.php	(revision 0)
@@ -0,0 +1,91 @@
+<?php
+header('Content-Type: application/atom+xml; charset=' . get_option('blog_charset'), true);
+echo '<?xml version="1.0" encoding="' . get_option('blog_charset') . '" ?' . '>';
+?>
+<feed
+	xmlns="http://www.w3.org/2005/Atom"
+	xml:lang="<?php echo get_option('rss_language'); ?>"
+	<?php do_action('atom_ns'); ?>
+>
+	<title type="text"><?php 
+		if (is_single() || is_page()) {
+			printf(__('Comments on: %s'), get_the_title_rss());
+		} else {
+			printf(__('Comments for %s'), get_bloginfo_rss('name'));
+		}
+	?></title>
+	<subtitle type="text"><?php bloginfo_rss('description'); ?></subtitle>
+	
+	<updated><?php echo mysql2date('Y-m-d\TH:i:s\Z', get_lastcommentmodified('GMT')); ?></updated>
+	<generator uri="http://wordpress.org/" version="<?php bloginfo('version'); ?>">WordPress</generator>
+	
+	<link rel="alternate" type="<?php bloginfo_rss('content_type'); ?>" content="<?php bloginfo_rss('home'); ?>" />
+	<link rel="self" type="application/atom+xml" href="<?php bloginfo_rss('comments_atom_url'); ?>" />
+	<id><?php bloginfo_rss('comments_atom_url'); ?></id>
+
+<?php
+$i = 0;
+if (have_posts()) :
+	while (have_posts()) : the_post();
+		if ($i < 1) {
+			$i++;
+		}
+		
+		if (is_single() || is_page()) {
+			$comments = $wpdb->get_results("SELECT comment_ID, comment_author, comment_author_email, 
+			comment_author_url, comment_date, comment_date_gmt, comment_content, comment_post_ID, 
+			$wpdb->posts.ID, $wpdb->posts.post_password FROM $wpdb->comments 
+			LEFT JOIN $wpdb->posts ON comment_post_id = id WHERE comment_post_ID = '" . get_the_ID() . "' 
+			AND $wpdb->comments.comment_approved = '1' AND $wpdb->posts.post_status = 'publish' 
+			AND post_date_gmt < '" . gmdate("Y-m-d H:i:59") . "' 
+			ORDER BY comment_date_gmt ASC" );
+		} else { // if no post id passed in, we'll just use the last posts_per_rss comments.
+			$comments = $wpdb->get_results("SELECT comment_ID, comment_author, comment_author_email, 
+			comment_author_url, comment_date, comment_date_gmt, comment_content, comment_post_ID, 
+			$wpdb->posts.ID, $wpdb->posts.post_password FROM $wpdb->comments 
+			LEFT JOIN $wpdb->posts ON comment_post_id = id WHERE $wpdb->posts.post_status = 'publish' 
+			AND $wpdb->comments.comment_approved = '1' AND post_date_gmt < '" . gmdate("Y-m-d H:i:s") . "'  
+			ORDER BY comment_date_gmt DESC LIMIT " . get_option('posts_per_rss') );
+		}
+		
+		if ($comments) {
+			foreach ($comments as $comment) {
+				$GLOBALS['comment'] =& $comment;
+				get_post_custom($comment->comment_post_ID);
+?>
+	<entry>
+		<title><?php
+			if (!(is_single() || is_page())) {
+				$title = get_the_title($comment->comment_post_ID);
+				$title = apply_filters('the_title', $title);
+				$title = apply_filters('the_title_rss', $title);
+				printf(__('Comment on %1$s by %2$s'), $title, get_comment_author_rss());
+			} else {
+				printf(__('By: %s'), get_comment_author_rss());
+			}
+		?></title>
+		<link rel="alternate" href="<?php comment_link(); ?>" type="<?php bloginfo_rss('content_type'); ?>" />
+		
+		<author>
+			<name><?php comment_author_rss(); ?></name>
+			<?php if (get_comment_author_url()) echo '<uri>' . get_comment_author_url() . '</uri>'; ?>
+		</author>
+		
+		<id><?php comment_link(); ?></id>
+		<updated><?php echo mysql2date('D, d M Y H:i:s +0000', get_comment_time('Y-m-d H:i:s', true), false); ?></updated>
+		<published><?php echo mysql2date('D, d M Y H:i:s +0000', get_comment_time('Y-m-d H:i:s', true), false); ?></published>
+		
+	<?php if (!empty($comment->post_password) && $_COOKIE['wp-postpass'] != $comment->post_password) { ?>
+		<content type="html" xml:base="<?php comment_link(); ?>"><![CDATA[<?php echo get_the_password_form(); ?>]]></content>
+	<?php } else { ?>
+		<content type="html" xml:base="<?php comment_link(); ?>"><![CDATA[<?php comment_text(); ?>]]></content>
+	<?php } ?>
+	</entry>
+<?php
+			}
+		}
+		
+	endwhile;
+endif;
+?>
+</feed>
\ No newline at end of file
Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 4927)
+++ wp-includes/functions.php	(working copy)
@@ -946,11 +946,16 @@
 
 	if ($feed == '' || $feed == 'feed')
     	$feed = 'rss2';
-
+	
 	$for_comments = false;
-	if ( 1 != get_query_var('withoutcomments') && ( is_singular() || get_query_var('withcomments') == 1 || $feed == 'comments-rss2' ) ) {
-		$feed = 'rss2';
-		$for_comments = true;	
+	if (get_query_var('withoutcomments') != 1 && (is_singular() || get_query_var('withcomments') == 1)) {
+		if ($feed == 'comments-rss2') {
+			$feed = 'rss2';
+			$for_comments = true;
+		} elseif ($feed == 'comments-atom') {
+			$feed = 'atom';
+			$for_comments = true;
+		}
 	}
 
 	$hook = 'do_feed_' . $feed;
@@ -973,8 +978,12 @@
 	}
 }
 
-function do_feed_atom() {
-	load_template(ABSPATH . WPINC . '/feed-atom.php');
+function do_feed_atom($for_comments) {
+	if ($for_comments) {
+		load_template(ABSPATH . WPINC . '/feed-atom-comments.php');
+	} else {
+		load_template(ABSPATH . WPINC . '/feed-atom.php');
+	}
 }
 
 function do_robots() {