The solution to add the 'template file' key to hook_views_api() does not appear to work in Drupal 7 yet. However, this does work, like a charm:

    /**
     * Implements hook_theme().
     */
    function bigtexas_theme() {
      return array(
        'views_view_fields__slideshow' => array(
        'variables' => array('view' => NULL, 'options' => NULL, 'row' => NULL),
        'template' => 'views-view-fields--slideshow',
        'base hook' => 'views_view_fields',
        'path' => drupal_get_path('module', 'bigtexas') . '/theme',
      ),
     );
    }