Index: comment-template.php
===================================================================
--- comment-template.php	(revision 11928)
+++ comment-template.php	(working copy)
@@ -21,14 +21,14 @@
  */
 function get_comment_author() {
 	global $comment;
-	if ( empty($comment->comment_author) ) {
-		if (!empty($comment->user_id)){
-			$user=get_userdata($comment->user_id);
-			$author=$user->user_login;
-		} else {
-			$author = __('Anonymous');
+	$author = __('Anonymous');
+	if (!empty($comment->user_id)){
+		$user=get_userdata($comment->user_id);
+		if($user)
+		{
+			$author=$user->display_name;
 		}
-	} else {
+	} else if(!empty($comment->comment_author)) {
 		$author = $comment->comment_author;
 	}
 	return apply_filters('get_comment_author', $author);