ciao, visto che la select non č multipla, puoi fare cos'

<script>
function upd_select(cucu){
if(cucu.notif.checked){
cucu.notifica.options[1].selected=true
}
else cucu.notifica.options[2].selected=true
}
</script>
<form name="form1">
<select name='notifica'>
<option value='niente'>-------</option>
<option value='ok'>Checkato</option>
<option value='no'>Non checkato</option>
</select>

<input type="checkbox" name="notif" onClick="upd_select(this.form)">
</form>