http://blog.darkhax.com/2009/05/28/twitter-tools-with-surl

surl.php

<?php
/*
Plugin Name: Tweet with surl
Plugin URI: http://s.darkhax.com
Description: Use surl with Twitter Tools
Version: 1.0
Author: Daniel Huckstep
Author URI: http://www.darkhax.com
*/
function get_content($url)
{
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_HEADER, 0);

    ob_start();

    curl_exec($ch);
    curl_close($ch);
    $string = ob_get_contents();

    ob_end_clean();

    return $string;
}

function surl($long_url) {
    return get_content("http://s.darkhax.com/?api&unsafe=on&url=".urlencode($long_url));
}

add_filter('tweet_blog_post_url', 'surl');

?>