The snippet below is a modification to Yoast’s excellent WordPress SEO plugin to include custom post type archives to the XML sitemaps it produces. Hope you find it useful.

            $post_type_archive_link = get_post_type_archive_link($post_type);
            if ($post_type_archive_link) {
                $url = array();
                $url['loc'] = $post_type_archive_link;
                $url['pri'] = 0.8;
                $url['chf'] = 'weekly';
                $this->write_sitemap_loc( $f, $url, $echo );
            }

Just add this snippet inside wp-content/plugins/wordpress-seo/sitemaps/xml-sitemap-class.php inside the function generate_sitemap. The code should go just inside the loop through the custom post types, e.g. after this chunk:

           if ( isset($options['post_types-'.$post_type.'-not_in_sitemap']) && $options['post_types-'.$post_type.'-not_in_sitemap'] )
                continue;
            if ( in_array( $post_type, array('revision','nav_menu_item','attachment') ) )
                continue;

Once you’ve added the code – just rebuild your Sitemap by visiting Seo » Dashboard » (Re)build XML Sitemap.