Mark Nottingham - SOAP: Protocol or Format?
"Contrast this with the simplicity of calling SOAP a format. There would be no SOAP protocol binding, just a format with a processing model.
If you wanted to transfer that format around using an existing protocol (like HTTP, SMTP, Jabber, etc.) you could; it's just a format that the person receiving it (including intermediaries, potentially) knows how to work with.
If you want to get fancy and describe your interfaces and interchanges in WSDL, you could without any fuss; as long as your underlying protocols were able to ship around XML, the only intrusion of SOAP would be the fact that you're using a particular format for your XML that has some implied processing semantics."