In case anyone was wondering (as I was) how to make the cart display on all pages, and not just on pages that are a "Product" or "Product Group" type page, here's how.
First of all you need to of course have the <% include Cart %> or <% include ProductMenu %> in your Page.ss template somewhere.
Then just add the following to the Page.php under the Page class...
/**
* Returns the shopping cart
*/
function Cart(){
return Order::ShoppingCart();
}