agregue esta linea <form action method="POST" name="formulario" id="formulario" onsubmit="tipo(); ">
Código HTML:
<script type="text/javascript"> function init() { document.getElementById("select1").disabled = true; } function tipo(radio){ document.getElementById("select1").disabled=(radio.value == 2)?true: false; } </script>
<body onload="init()">
<table>
<tr>
<th>Tipo</th>
<td>
<input type="radio" name="tipo" value="1" onclick="tipo(this)" />Uno
<input type="radio" name="tipo" value="2" checked onclick="tipo(this)" />Ninguno
</td>
</tr>
<tr>
<th>Nombre</th>
<td>
<select name="select1" id="select1">
<option value="1">ABC</option>
<option value="2">DFG</option>
</select>
</td>
</tr>
</table>