=== modified file 'includes/menu.inc'
--- includes/menu.inc	2010-11-07 21:51:40 +0000
+++ includes/menu.inc	2010-11-14 20:27:35 +0000
@@ -1207,6 +1207,7 @@ function menu_tree_page_data($menu_name,
           $tree_parameters['expanded'] = $active_trail;
           $tree_parameters['active_trail'] = $active_trail;
         }
+        drupal_alter('menu_tree_active_trail', $tree_parameters, $cid);
         // Cache the tree building parameters using the page-specific cid.
         cache_set($cid, $tree_parameters, 'cache_menu');
       }

=== modified file 'modules/system/system.api.php'
--- modules/system/system.api.php	2010-11-13 02:00:56 +0000
+++ modules/system/system.api.php	2010-11-14 20:32:45 +0000
@@ -1186,6 +1186,25 @@ function hook_translated_menu_link_alter
 }
 
 /**
+ * Alter the current page menu tree parameters like the active trail.
+ *
+ * While menu_set_active_trail() can be used to set the active trail it only
+ * affects the breadcrumb. This alter can be used to change the menu link
+ * ids stored in $tree_parameters['active_trail'] indicating which links in
+ * the page menu should be open.
+ *
+ * @param $tree_parameters
+ *   See menu_build_tree() for the description of this array.
+ * @param $cid
+ *   The cache id for these parameters. Add your identifiers separated by
+ *   colons as necessary.
+ */
+function hook_menu_tree_active_trail_alter(&$tree_parameters, &$cid) {
+  // Only show the three levels of the menu.
+  $tree_parameters['max_depth'] = 3;
+}
+
+/**
  * Inform modules that a menu link has been created.
  *
  * This hook is used to notify modules that menu items have been