Finally something that stays/pays on same page. Great job QT.
Question: is this for X-Payments only? Some gateways have their own iframe which currently loads on separate page - will that change? so these payment gateways iframes load on the same OPC without using X-Payments?
Suggestion: re-organize this page. You switched "payment methods" block with "coupons" block and it seems it messes up the pay-flow process. "order summary" block is before "payment methods" block which doesn't seem right. Something like this may work way better
http://www.cflsystems.com/public/everyone/opc-1.jpg
http://www.cflsystems.com/public/everyone/opc-2.jpg