Additional submit handler for the 'Edit field instance' form.
Make sure the 'user_register_form' setting is set for required fields.
function user_form_field_ui_field_edit_form_submit($form, &$form_state) {
$instance = $form_state['values']['instance'];
if (!empty($instance['required'])) {
form_set_value($form['instance']['settings']['user_register_form'], 1, $form_state);
}
}