Only displays WordPress posts from the last 30 days. Also works with custom post types.

only-new.php

<?php
function filter_where($where = '') {
    //posts in the last 30 days
    $where .= " AND post_date > '" . date('Y-m-d', strtotime('-30 days')) . "'";
    return $where;
}
add_filter('posts_where', 'filter_where');
query_posts($query_string);
?>