function yoast_bitly_shortlink($url, $id, $context, $allow_slugs) {
    if ( ( is_singular() && !is_preview() ) || $context == 'post' ) {
        $short = get_post_meta($id, '_yoast_bitlylink', true);
        if ( !$short || $short == '' ) {
            if ( !defined('BITLY_USERNAME') || !defined('BITLY_APIKEY') ) {
                $short = 'http://yoast.com/wordpress/bitly-shortlinks/configure-bitly/';
            } else {
                $url = get_permalink( $id );
                $resp = wp_remote_get('http://api.bit.ly/v3/shorten?format=txt&longUrl='.$url.'&login='.BITLY_USERNAME.'&apiKey='.BITLY_APIKEY);
                if (is_array($resp))
                    if ( $resp['response']['code'] == 200 ) {
                        $short = $resp['body'];
                        update_post_meta( $id, '_yoast_bitlylink', $short);
                    }
            }
        }
        return $short;
    }
    return false;
}
add_filter('pre_get_shortlink','yoast_bitly_shortlink',99,4);