Make featured image required

Fairly simple using jQuery and global $typenow ex:

add_action('admin_print_scripts-post.php', 'my_publish_admin_hook');
add_action('admin_print_scripts-post-new.php', 'my_publish_admin_hook');
function my_publish_admin_hook(){
    global $typenow;
    if (in_array($typenow, array('post','page','mm_photo '))){
        ?>
        <script language="javascript" type="text/javascript">
            jQuery(document).ready(function() {
                jQuery('#post').submit(function() {
                    if (jQuery("#set-post-thumbnail").find('img').size() > 0) {
                        jQuery('#ajax-loading').hide();
                        jQuery('#publish').removeClass('button-primary-disabled');
                        return true;
                    }else{
                        alert("please set a featured image!!!");
                        jQuery('#ajax-loading').hide();
                        jQuery('#publish').removeClass('button-primary-disabled');
                        return false;
                    }
                    return false;
                });
            });
        </script>

        <?php
    }
}