Cambio ACTION di un form in explorer 6

[H5N1_Aviaria]

<script type="text/javascript">
function cambiaAction(){
document.getElementById('form').action='index.php';
alert(document.getElementById('form').action); /*verifica debug*/
}
</script>

<form id="form" action=""><fieldset>
<input type="button" onclick="cambiaAction()" />
<input type="submit" /></fieldset>
</form> 

A me in IE6 funziona...