$form['field']["bio_registration_$field_name"] = array(
      '#type' => 'checkboxes',
      '#title' => t('Registration form'),
      '#options' => array(1 => t('Visible in user registration form')),
    );
if (variable_get("bio_registration_$field_name", 0)) { $form['field']['whatever']['#default_value'][] = 1};