We have a fairly pressing need to be able to track the currency with an eCommerce order. We receive purchases in many currencies on the site and at the moment they are all recorded in the Piwik site currency which causes large discrepancies with the actual value of the orders. I appreciate that to reconcile the different currencies would require some kind of an exchange rate but this should at least be exposed for development of a plugin if not in the core Piwik code.