see this page:
http://www.worldpay.com/support/bg/index.php?page=news&sub=worldpay-url-changes&c=WW
world pay recently changed the URL for the gateway. you should use:
https://secure.worldpay.com/wcc/purchase
as the forms action. I believe that you will also need to set the account to accept live transactions in addition to setting the test mode variable to 0.