<form name="f" method="post" action="#">
	<fieldset>
	<legend>Validao de CNPJ</legend>
	<label>CNPJ:</label>
	<input type="text" name="cnpj" />
	<input type="submit" value="Checar" />
	</fieldset>
</form>

<script type="text/javascript">
//<![CDATA[

addEvent(document.forms.f, "submit", function(e){
	alert(this.cnpj.value.isCNPJ() ? "Vlido" : "Incorreto");
	e.preventDefault();
});

//]]>
</script>