Update warning: "Some of the pending updates cannot be applied because their dependencies were not met." Followed by "No pending updates."

Just in case someone else finds this ... I added a watchdog line to the function update_script_selection_form($form, &$form_state) in update.php.

foreach ($updates as $module => $update) {
  if (!isset($update['start'])) {
    $form['start'][$module] = array(
      '#type' => 'item',
      '#title' => $module . ' module',
      '#markup'  => $update['warning'],
      '#prefix' => '<div class="messages warning">',
      '#suffix' => '</div>',
    );
    watchdog('update','NO START: '.$module);
    $incompatible_updates_exist = TRUE;

    ...

It was XML sitemap causing the problems on a D6->D7 upgrade, and I ended up having to delete its tables manually before it would uninstall.