[Plugin: WyPiekacz] Require featured image depending on category

In case any one is interested, here’s a way to require featured images by category using Wypiekacz

function check_my_custom_post_thumbnail( $check, $post_id, $post_data ) {
  if ( $post_id <= 0 ) {
    return false;
  }

  $categories = array(); //categories set by editor
  if (is_array($post_data)) {
    $categories = $post_data['post_category'];
  } elseif (is_object($post_data)) {
    $categories = $post_data->post_category;
  }

  $categories_requiring_image = array(4); //array containing IDs of the categories for which you want to require a featured image

  if (is_array($categories) && count(array_intersect($categories_requiring_image, array_values($categories)))) {
    $image = get_the_image( array(
            'post_id' => $post_id,
            'attachment' => false,
            'echo' => false,
            ) );
    return !empty($image);
  } else {
    return true;
  }
}
add_filter( 'wypiekacz_check_thumbnail', 'check_my_custom_post_thumbnail', 10, 3);

http://wordpress.org/extend/plugins/wypiekacz/