Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.1159
diff -u -p -r1.1159 common.inc
--- includes/common.inc	1 May 2010 08:12:22 -0000	1.1159
+++ includes/common.inc	3 May 2010 17:07:03 -0000
@@ -4834,6 +4834,13 @@ function drupal_pre_render_markup($eleme
 function drupal_render_page($page) {
   $main_content_display = &drupal_static('system_main_content_added', FALSE);
 
+  // menu_execute_active_handler() initializes the theme before invoking the
+  // page callback, but it's possible that drupal_render_page() is called for
+  // an error page (MENU_NOT_FOUND, MENU_ACCESS_DENIED) or during hook_init().
+  // Even in these situations, the theme may implement hook_page_alter() and
+  // other alter hooks that get called during hook_page_build().
+  drupal_theme_initialize();
+
   // Allow menu callbacks to return strings or arbitrary arrays to render.
   // If the array returned is not of #type page directly, we need to fill
   // in the page with defaults.
Index: includes/menu.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/menu.inc,v
retrieving revision 1.388
diff -u -p -r1.388 menu.inc
--- includes/menu.inc	1 May 2010 08:12:22 -0000	1.388
+++ includes/menu.inc	3 May 2010 17:07:04 -0000
@@ -466,6 +466,9 @@ function menu_execute_active_handler($pa
         if ($router_item['include_file']) {
           require_once DRUPAL_ROOT . '/' . $router_item['include_file'];
         }
+        // The theme may implement alter hooks to be called during page callback
+        // execution prior to it otherwise initialized.
+        drupal_theme_initialize();
         $page_callback_result = call_user_func_array($router_item['page_callback'], $router_item['page_arguments']);
       }
       else {