A little helper function for Drupal 6 form manipulation

_custom_attributes_add_class.php

<?php

/**
 * Adds a class to a form element.
 */
function _custom_attributes_add_class($element, $class) {
  if ((!is_array($element)) || ((int)strlen($class) === (int)0)) return $element;
  
  if (isset($element['#attributes'])) {
    if (!is_array($element['#attributes'])) return $element;
    if (isset($element['#attributes']['class'])) {
      $element['#attributes']['class'] .= ' ' . $class;
    }
    else {
      $element['#attributes']['class'] = $class;
    }
  }
  else {
    $element['#attributes'] = array();
    $element['#attributes']['class'] = $class;
  }
  return $element;
}

/**
 * Implements HOOK_form_alter.
 * Example usage.
 */
function mymodule_form_alter(&$form, $form_state, $form_id) {
  $form['buttons']['submit'] = _custom_attributes_add_class($form['buttons']['submit'], 'custom');
}