Try refreshing the Cufont font each time the drop down changes. Like this:
$(document).ready(function(){
var select = $("select");
select.selectBoxIt();
refreshCufon();
select.change(function() {
refreshCufon();
});
function refreshCufon() {
Cufon.replace('input.btn-submit', { fontFamily: 'Myriad Pro' });
Cufon.refresh();
}
});
</script>