Found it - this code works perfectly:
add_filter( 'woocommerce_product_tabs', 'woo_reorder_tabs', 98 );
function woo_reorder_tabs( $tabs ) {
$tabs['additional_information']['priority'] = 5; // Additional information first
$tabs['description']['priority'] = 10; // Description second
$tabs['reviews']['priority'] = 15; // Reviews last
return $tabs;
}More on editing product data tabs in the official WooCommerce documentation: http://docs.woothemes.com/document/editing-product-data-tabs/
Cheers!