This plugin adds a sale items shortcode to WooCommerce. Usage:
[sale_products]
Options & defaults:
per_page=12
columns=4
orderby=title
order=asc
This code was taken directly from the shortcode-init.php file in WooCommerce 1.6.4 and modified to output sale items instead of featured items. It's been submitted to the Woocommerce git repo, so hopefully it will be added soon and you won't need a separate plugin.