...così funziona...

<script type="text/javascript">
function Attiva(){
var sel = document.getElementById('Categoria');
if ((sel.options[sel.selectedIndex].value != '1') && (sel.options[sel.selectedIndex].value != '2')) {
document.getElementById('Sub_Categoria').style.dis play = "block";
return "block";
}
else {
document.getElementById('Sub_Categoria').style.dis play = "none";
return "none";
}
}
</script>

<select name="Categoria" id="Categoria" onchange="Attiva();">
<option value="-1" selected="selected">- Seleziona -</option>
<option value="1">A</option>
<option value="2">B</option>
<option value="3">C</option>
</select>

<select name="Sub_Categoria" size="1" id="Sub_Categoria" style="display:none">
<option value="-1" selected="selected">- Seleziona -</option>
<option value="1">Prova 1</option>
<option value="2">Prova 2</option>
</select>