//Hack found on the bottom of http://codex.wordpress.org/Template_Tags/query_posts
function filter_where($where = '') {
//posts in the last 7 days
$where .= " AND post_date > '" . date('Y-m-d', strtotime('-7 days')) . "'";
}
add_filter('posts_where', 'filter_where');
query_posts($query_string);
if ( have_posts() ) : while ( have_posts() ) : the_post();
... and the usual