From `/wp-includes/widgets.php` for the `WP_Category_Widget` class we have the following code *(line 438 in WordPress v3.0.4):*

    $cat_args = array('orderby' => 'name', 'show_count' => $c, 'hierarchical' => $h);
    if ( $d ) {
      $cat_args['show_option_none'] = __('Select Category');
  
    wp_dropdown_categories(
      apply_filters('widget_categories_dropdown_args',$cat_args)
    );

From that I would surmise the hook `'widget_categories_dropdown_args'` should allow you to set an `'orderby'` parameter?





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  [1]: http://www.jetbrains.com/phpstorm/