Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 19622)
+++ wp-includes/functions.php	(working copy)
@@ -1134,7 +1134,6 @@
 	} else {
 		$site_url = parse_url( site_url() );
 		$path = ( !empty( $site_url['path'] ) ) ? $site_url['path'] : '';
-		$output .= "Disallow: $path/wp-admin/\n";
 		$output .= "Disallow: $path/wp-includes/\n";
 	}
 
@@ -3528,6 +3527,18 @@
 }
 
 /**
+ * Send a HTTP header to prevent indexation of opened URL by search engines that support it.
+ *
+ * @link http://googleblog.blogspot.com/2007/07/robots-exclusion-protocol-now-with-even.html
+ *
+ * @since 3.3.0
+ * @return none
+ */
+function send_noindex_header() {
+	@header("X-Robots-Tag: noindex");
+}
+
+/**
  * Returns a MySQL expression for selecting the week number based on the start_of_week option.
  *
  * @internal
Index: wp-admin/admin-ajax.php
===================================================================
--- wp-admin/admin-ajax.php	(revision 19622)
+++ wp-admin/admin-ajax.php	(working copy)
@@ -14,6 +14,8 @@
 define('DOING_AJAX', true);
 define('WP_ADMIN', true);
 
+send_noindex_header();
+
 if ( ! isset( $_REQUEST['action'] ) )
 	die('-1');
 
Index: wp-admin/admin.php
===================================================================
--- wp-admin/admin.php	(revision 19622)
+++ wp-admin/admin.php	(working copy)
@@ -66,6 +66,7 @@
 auth_redirect();
 
 nocache_headers();
+send_noindex_header();
 
 // Schedule trash collection
 if ( !wp_next_scheduled('wp_scheduled_delete') && !defined('WP_INSTALLING') )