codice:
function goThere(s){
	var optValue=s.options[s.selectedIndex].value;
	if(optValue!=""){parent.centro.location.href=optValue;}
}

<form name="menu">
	<select name="selectThis" size="1" onchange="goThere(this);">
	<option selected="selected" value="">Naviga il sito</option>
	<option value="http://www.ciao.it">ddddd</option>
	</select>
</form>