Mine

    if (count($db_types) > 1) {
      // Database type
      $db_types = drupal_detect_database_types();
var_dump($db_type); # NULL
var_dump($db_types); # array(3) { ["mysql"]=>  string(5) "mysql" ["mysqli"]=>  string(6) "mysqli" ["pgsql"]=>  string(5) "pgsql" } 
      $form['basic_options']['db_type'] = array(
        '#type' => 'radios',
        '#title' => st('Database type'),
        '#required' => TRUE,
        '#options' => $db_types,
        '#default_value' => $db_type ? $db_type : 'mysql', # wtf??
        '#description' => st('The type of database your @drupal data will be stored in.', array('@drupal' => drupal_install_profile_name())),
      );
      $db_path_description = st('The name of the database your @drupal data will be stored in. It must exist on your server before @drupal can be installed.', array('@drupal' => drupal_install_profile_name()));
    }