Add this code to the head of your form. Edit the #item1_text_1 to fit your forms input id.
<script src="common/libs_js/jquery-1.4.4.min.js"></script>
<script>
$(document).ready(function () {
$(':checkbox').click(function() {
$('#item1_text_1').val(
$(':checkbox:checked').map(function() {
return $(this).val();
}).get().join(',')
);
});
});
</script>