actions.class.php

<?php

class userActions extends sfActions
{
  public function executeCheckUsername(sfWebRequest $request)
  {
    $username     = $request->getParameter('username');
    $availability = (bool) Doctrine_Core::getTable('User')->checkUsername($username);

    return $this->renderText(json_encode($availability);
  }
}

formSuccess.php

<script type="text/javascript">
  $.get(
    '<?php echo url_for('ajax_check_username'); ?>',
    {
      username: $('input#username').val()
    },
    function(data)
    {
      // do stuff here
    }
  );
</script>

routing.yml

ajax_check_username:
  url:   /user/checkusername
  param: { module: user, action: checkUsername }