Just thought I'd add to this guys, I have had callback issues with the Worldpay module in the past as well. I am using it on a site I am finishing off at the moment, and I am a bit concerned about the callback issues as I can't test it properly until it is actually live. I think if I remember right, Worldpay does callback but it doesn't always update the cart, and so there's not always a record of payment in the admin. Last time I had these issues I think it just went to a blank page on callback.