Ignoriamo possibili personalizzazioni future,
a me non funziona proprio questo:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Documento senza titolo</title>
</head>

<body>
<script language="javascript">

function funz(nomeText, nomeSelect){
document.nomeForm.elements[nomeText].value =
document.nomeForm.elements[nomeSelect].options[document.nomeForm.elements[nomeSelect].selectedIndex].value;
}

</script>


<form name="nomeForm">

<select name="XXX" onChange="funz(ZZZ, XXX)">
<option value="valore 1">1</option>
<option value="valore 2">2</option>
<option value="valore 3">3</option>
</select>

<input name="ZZZ" type="text">

</form>

</body>
</html>


Mi compaiono i due box, ma la selezione non produce alcun risultato...