Normally when there is a form input error, Drupal highlights the form input by adding a red border and changing the text color. Unfortunately, nothing happens for checkboxes and radio buttons. Here's a little jQuery to add the error class to the label directly above a set of checkboxes or radios when an error occurs.
<?php
$('form#node-form div.form-item').each(function(){
var result = 0;
result += $(this).find('div.form-radios input.form-radio.error').length;
result += $(this).find('div.form-checkboxes input.form-checkbox.error').length;
if (result) $(this).find('label:first').removeClass('error').addClass('error');
});
?>