#108971, use any menu not just root menus, patch by ewhipple

simplemenu.module

@@ -58,7 +58,7 @@ function simplemenu_admin_settings() {
   $form['default_menu']['simplemenu_menu'] = array(
     '#type' => 'select',
     '#title' => t('Menu'),
-    '#options' => menu_get_root_menus(),
+    '#options' => menu_parent_options(0),
     '#default_value' => variable_get('simplemenu_menu', 1),
     '#description' => t('Select the menu to display.')
   );