codice:
<html>
<head>
<title>Prova</title>
<script type="text/javascript">
function copy_model(){
    document.nomeform.new_model.value=document.nomeform.db_model[document.nomeform.db_model.selectedIndex].value;
}
</script>
</head>
<body>
<form name="nomeform">
<select name="db_model" onchange="copy_model()">
    <option value="">Seleziona un valore</option>
    <option value="uno">uno</option>
    <option value="due">due</option>
    <option value="tre">tre</option>
    <option value="quattro">quattro</option>
</select>


Hai selezionato: <input type="text" name="new_model" />
</form>
</body>
</html>
HTH
Zappa