Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.146
diff -u -r1.146 system.admin.inc
--- modules/system/system.admin.inc	16 May 2009 20:10:50 -0000	1.146
+++ modules/system/system.admin.inc	19 May 2009 04:26:15 -0000
@@ -1228,6 +1228,12 @@
     '#description' => t('This page is displayed when no other content matches the requested document. If unsure, specify nothing.'),
     '#field_prefix' => url(NULL, array('absolute' => TRUE)) . (variable_get('clean_url', 0) ? '' : '?q=')
   );
+  $form['site_error_show_blocks'] = array(
+    '#type' => 'checkbox',
+    '#title' => t('Display blocks on 403/404 pages'),
+    '#default_value' => '',
+    '#description' => t('When enabled, will show the blocks on the not found/access denied pages. Take caution, as this can effect the performance of your site.'),
+  );
   $form['#validate'][] = 'system_site_information_settings_validate';
 
   return system_settings_form($form);
Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.904
diff -u -r1.904 common.inc
--- includes/common.inc	16 May 2009 19:58:38 -0000	1.904
+++ includes/common.inc	19 May 2009 04:26:14 -0000
@@ -370,13 +370,18 @@
     // Standard 404 handler.
     drupal_set_title(t('Page not found'));
     $return = t('The requested page could not be found.');
-  }
 
-  $page = drupal_get_page($return);
-  // To conserve CPU and bandwidth, omit the blocks.
-  $page['#show_blocks'] = FALSE;
+    // Show the blocks from the front page if desired.
+    if (variable_get('site_error_show_blocks')) {
+      menu_set_active_item(variable_get('site_frontpage', 'node'));
+    }
+  }
+  else {
+    // The menu path was a success. Remove the blocks if desired to conserve CPU.
+    $return['#show_blocks'] = variable_get('site_error_show_blocks');
+  }
 
-  print drupal_render_page($page);
+  print drupal_render_page($return);
 }
 
 /**
@@ -399,6 +404,15 @@
     // Standard 403 handler.
     drupal_set_title(t('Access denied'));
     $return = t('You are not authorized to access this page.');
+
+    // Show the blocks from the front page if desired.
+    if (variable_get('site_error_show_blocks')) {
+      menu_set_active_item(variable_get('site_frontpage', 'node'));
+    }
+  }
+  else {
+    // The menu path was a success. Remove the blocks if desired to conserve CPU.
+    $return['#show_blocks'] = variable_get('site_error_show_blocks');
   }
 
   print drupal_render_page($return);