Index: includes/menu.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/menu.inc,v
retrieving revision 1.330
diff -u -p -r1.330 menu.inc
--- includes/menu.inc	20 Jul 2009 01:28:16 -0000	1.330
+++ includes/menu.inc	29 Jul 2009 17:33:40 -0000
@@ -406,6 +406,11 @@ function menu_execute_active_handler($pa
       return MENU_ACCESS_DENIED;
     }
   }
+  $current_path = isset($path) ? $_GET['q'] : NULL;
+  $page_not_found = module_invoke_all('page_not_found', $current_path);
+  if (isset($page_not_found) && is_array($page_not_found) && isset($page_not_found['status_code'])) {
+    return $page_not_found['status_code'];
+  }
   return MENU_NOT_FOUND;
 }
 
@@ -1269,7 +1274,7 @@ function theme_menu_tree($tree) {
  * The menu item's LI element is given one of the following classes:
  * - expanded: The menu item is showing its submenu.
  * - collapsed: The menu item has a submenu which is not shown.
- * - leaf: The menu item has no submenu. 
+ * - leaf: The menu item has no submenu.
  *
  * @ingroup themeable
  *
Index: modules/menu/menu.api.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/menu/menu.api.php,v
retrieving revision 1.9
diff -u -p -r1.9 menu.api.php
--- modules/menu/menu.api.php	11 Jul 2009 13:56:21 -0000	1.9
+++ modules/menu/menu.api.php	29 Jul 2009 17:33:41 -0000
@@ -148,5 +148,36 @@ function hook_translated_menu_link_alter
 }
 
 /**
+ * Execute this hook when the current path would result in a 'page not found'
+ * response.
+ *
+ * This hook may be used, for example, to load balance files located on
+ * different servers, or to check a subdomain for a path to redirect to.
+ *
+ * The menu return status can be changed by returning an array with the key
+ * 'status_code' set to the desired constant.
+ *
+ * @see menu.inc
+ *
+ * @param $path
+ *   The system path which was not found.
+ *
+ * @return
+ *   An optional array containing a 'status_code' key corresponding to a menu
+ *   return constant from menu.inc.
+ */
+function hook_page_not_found($path) {
+  // Example - determine if the URL is available at a different site, and
+  // redirect if that URL is available.
+  $remote_url = variable_get('offsite_redirect_site', FALSE);
+  if ($remote_url) {
+    $result = drupal_http_request($remote_url . $path);
+    if (floor($result->code / 100) * 100 == 200 || floor($result->code / 100) * 100 == 300) {
+      drupal_goto($remote_url . $path);
+    }
+  }
+}
+
+/**
  * @} End of "addtogroup hooks".
  */