Autocomplete.php

<?php use_helper('Javascript') ?>

<?php echo javascript_include_tag('/sf/prototype/js/prototype.js') ?>
<?php echo javascript_include_tag('/js/javascript.js') ?>
<?php echo javascript_include_tag('/sf/sf_web_debug/js/main.js') ?>
<?php echo javascript_include_tag('/sf/prototype/js/builder.js') ?>


<?php echo input_auto_complete_tag('Companies', '', 'autoCompleta/buscarUsuario',
                                array('autocomplete' => 'on', 'onClick' => 'this.select();'),
                                array('use_style' => true, 'after_update_element'  => "function (inputField, selectedItem) { $('object_id').value = selectedItem.id; }")) ?>


action.class.php
class autoCompletaActions extends sfActions
{
    public function executeBuscarUsuario()
    {
        /*
       $letras = $this->getRequestParameter('UsuarioApellido');
       $c = new Criteria();
       $c->add(UsuarioPeer::APELLIDO,"${letras}%",Criteria::LIKE);
       $this->usuarios = UsuarioPeer::doSelect($c);
       */
    }
}

tempalte
<ul>
<li>Uno</li>
<li>Dos</li>
<li>Tres</li>
</ul>