Brad,
I do plan to implement it as a module, so I am sure that I will need to take you up on your offer of answering some questions as I progress.
Bart,
I will look through the two c libraries (SecPay and CyberSource) and will propose some additions to the payment gateway service contract. Perhaps then, once that has survived scrutiny I can set about writing an implementation.
Am I better to use the Authorize.net Gateway as a guide or stick to the Payflow one?
Richard