prova cosi'
codice:
<form>
<select onchange="this.form.disabilitato.disabled=(this.selectedIndex!=0)?false:true">
<option>scegli:</option>
<option value="pinco">pinco</option>
<option value="pallino">pallino</option>
</select>
<input type="button" name="disabilitato" value="vai" disabled="disabled" />
</form>
ciao