If you have Tweenest —http://pongsocket.com/tweetnest/ — installed, then the following two functions will pull that data in to your WordPress install. This code assumes that the default table prefix is used (tn_).

gistfile1.php

<?php
function get_tweetnest_tweets($args) {
  global $wpdb; 
	
	$args = shortcode_atts(array(
		'count' => 5,
	), $args);

	$sql	= sprintf("SELECT `tn_tweets`.*, `tn_tweetusers`.`screenname`, `tn_tweetusers`.`realname`, `tn_tweetusers`.`profileimage`
					   FROM `tn_tweets`
					   LEFT JOIN `tn_tweetusers` ON `tn_tweets`.`userid` = `tn_tweetusers`.`userid`
					   ORDER BY `tn_tweets`.`time` DESC
					   LIMIT %d", $args['count']);
	
	$tweets		= $wpdb->get_results($sql);
	$content	= array();
	
	foreach($tweets as $tweet) {
		$html	= wptexturize($tweet->text);
		$href	= esc_url(get_twitter_status_url($tweet->screenname, $tweet->tweetid));
		$time	= sprintf('%s ago', human_time_diff($tweet->time));
		$content[]	= sprintf('<li>%s <a href="%s" class="tweet_time" rel="external">%s</a></li>', $html, $href, $time);
	}
	
	return sprintf('<div class="tweets"><ul>%s</ul></div>', implode("\r\n", $content));
}
function get_twitter_status_url($username, $id) {
	return sprintf('http://twitter.com/%s/status/%s', urlencode($username), urlencode($id));
}