Set WordPress shortlink to siteurl.com/1234

// By Clifford Paulick of http://tourkick.com/
//
// RESULTS IN SETTING http://siteurl.com/1234 AS THE SHORTLINK
//
// MAKE SURE YOU ENABLE THESE LINKS TO WORK VIA SOME METHOD, like http://managewp.com/?p=377184
//
function post_id_shortlink() {
global $post;
    if ( is_singular() ) {
        $postid = $post->ID;
            $idshortlink = site_url() . '/' . $postid;
            return $idshortlink;
        }
    }
add_filter( 'pre_get_shortlink', 'post_id_shortlink', 99); // priority 99 overrides bit.ly and other plugin's default priority of 10