By modifying line 406 of product_display_functions.php in the WP e-Commerce plugin, I was able to get the "Additional Description" on a product to filter for shortcodes.
$output .= do_shortcode(wpautop(stripslashes($product['additional_description']))) . "";
Ideas about how to turn this into a simple plugin? As it is now, if the plugin is updated, my hack goes away.