Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.920
diff -u -p -r1.920 common.inc
--- includes/common.inc	8 Jun 2009 05:00:11 -0000	1.920
+++ includes/common.inc	8 Jun 2009 14:20:11 -0000
@@ -378,8 +378,8 @@ function drupal_not_found() {
 
   drupal_set_page_content($return);
   $page = element_info('page');
-  // To conserve CPU and bandwidth, omit the blocks.
-  $page['#show_blocks'] = FALSE;
+  // Optionally omit the blocks to conserve CPU and bandwidth.
+  $page['#show_blocks'] = variable_get('site_404_blocks', FALSE);
 
   print drupal_render_page($page);
 }
Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.155
diff -u -p -r1.155 system.admin.inc
--- modules/system/system.admin.inc	8 Jun 2009 04:48:43 -0000	1.155
+++ modules/system/system.admin.inc	8 Jun 2009 14:22:36 -0000
@@ -1224,7 +1224,7 @@ function system_site_information_setting
     '#title' => t('Default 403 (access denied) page'),
     '#default_value' => '',
     '#size' => 40,
-    '#description' => t('This page is displayed when the requested document is denied to the current user. If unsure, specify nothing.'),
+    '#description' => t('This page is displayed when the requested document is denied to the current user. Leave blank to display a generic "access denied" page.'),
     '#field_prefix' => url(NULL, array('absolute' => TRUE)) . (variable_get('clean_url', 0) ? '' : '?q=')
   );
   $form['site_404'] = array(
@@ -1232,9 +1232,15 @@ function system_site_information_setting
     '#title' => t('Default 404 (not found) page'),
     '#default_value' => '',
     '#size' => 40,
-    '#description' => t('This page is displayed when no other content matches the requested document. If unsure, specify nothing.'),
+    '#description' => t('This page is displayed when no other content matches the requested document. Leave blank to display a generic "page not found" page.'),
     '#field_prefix' => url(NULL, array('absolute' => TRUE)) . (variable_get('clean_url', 0) ? '' : '?q=')
   );
+  $form['site_404_blocks'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Display blocks on default 404 (not found) page'),
+    '#description' => t('Render all blocks on the default 404 (not found) page. Disabling blocks can help with performance but might leave users with a less functional site.'),
+    '#default_value' => variable_get('site_404_blocks', 0)
+  );
   $form['#validate'][] = 'system_site_information_settings_validate';
 
   return system_settings_form($form);