Add Custom Post Types to the feed

add_filter('request', 'add_ctp_feed');
function add_ctp_feed($qv) {
    if (isset($qv['feed']) && !isset($qv['post_type']))
// 'event' and 'press' are sample custom post types
        $qv['post_type'] = array('post', 'event', 'press');
    return $qv;
}