Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 4545)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -181,7 +181,7 @@
 	$key = $_POST['meta'][$mid]['key'];
 	$value = $_POST['meta'][$mid]['value'];
 	if ( !$meta = get_post_meta_by_id( $mid ) )
-		die('0');
+		die('0'); // if meta doesn't exist
 	if ( !current_user_can( 'edit_post', $meta->post_id ) )
 		die('-1');
 	if ( $u = update_meta( $mid, $key, $value ) ) {
@@ -195,7 +195,7 @@
 		) );
 		$x->send();
 	}
-	die('0');
+	die('1'); // We know meta exists; we also know it's unchanged (or DB error, in which case there are bigger problems).
 	break;
 case 'add-user' :
 	if ( !current_user_can('edit_users') )