// Check category or query
    $v = new sfValidatorCallback(array(
      'callback' => array($this, 'checkCategoryOrQuery')
    ,) ,
    array(
      'invalid'  => 'Veuillez choisir une rubrique ou un mot clé'
    ));
 
    $this->validatorSchema->setPostValidator($v);

et le callback qui va bien:

  /**
   * Check if a category or a keyword has been set.
   *
   * @param sfValidator $validator
   * @param Array $value Form values
   * @param Array $arguments  Validation arguments
   * 
   * @return mixed
   */
  public function checkCategoryOrQuery($validator, $value, $arguments)
  {
    if (empty($value['category_id']) && empty($value['nature_id']))
    {
      throw new sfValidatorError($validator, 'invalid');
    }
 
    return $value;
  }