I've posted a new loooong article about Electronic Payment Processing with ASP.NET at:
http://www.west-wind.com/presentations/aspnetecommerce/aspnetecommerce.asp
The article covers a lot of ground from an overview of the whole process, as well as specific code implementations for several common gateways including Authorize.NET, PayFlowPro and LinkPoint and a couple of others. There's also a walk through of how this stuff integrates into a live application. There's a lot of detail so the article is long <g>.
This topic acts as a placeholder for the article. If you have any comments, questions etc about the article please leave them here.