Código PHP:
<script>
select.onchange = function() {
boton.disabled = (select.selectedIndex==-1);
}
</script>
<select name="select" id="select">
<option value="" selected>Seleccione</option>
<option value="1" selected>Seleccione</option>
<option value="2" selected>Seleccione</option>
<option value="3" selected>Seleccione</option>
</select>
<button id="boton">nada</button>
???