Mine

<?php

function poof_menu() {
  $items['poof'] = array(
    'title' => 'Ahah!',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('poof_form'),
    'access arguments' => array('access content')
  );
  $items['poof/message_js'] = array(
    'page callback' => 'poof_message_js',
    'type' => MENU_CALLBACK,
    'access arguments' => array('access content')
  );
  return $items;
}

function poof_form() {
  $form['target'] = array(
    '#type' => 'markup',
    '#prefix' => '<div id="target">Click me. I dare you.'</div>',
  );
  $form['submit'] = array(
    '#type' => 'button',
    '#value' => t('Click Me'),
    '#ahah' => array(
      'event' => 'click',
      'path' => 'poof/message_js',
      'wrapper' => 'target',
      'effect' => 'fade',
    )
  );

  return $form;
}

function poof_message_js() {
  $output = t('POOF!');
  drupal_json(array('status' => TRUE, 'data' => $output));
}