New posts label category with “new”

In case anyone else is looking for this in the future, turns out the best way to do it is by using wp_get_recent_posts function. Someone knocked up an example that worked great for me:

<?php $args = array( 'numberposts' => '1' );
$recent_post = wp_get_recent_posts( $args );

if (strtotime($recent_post[0]['post_date']) > strtotime('-7 days'))
    $class = "new!";
?>