In response to Dirk's question (some time ago).
Personally, I model the ESB as an intermediate application component with internal functions implementing services. The services are the real ones that are published by the ESB. The consumer application will use the ESB functions from the same services that they are implementing. For sure the function is the ESB is generally limited and the signature of the web service in the ESB is the same than the signature of the web service of the real server it points to (if the ESB is doing some processing or orchestration, it works also).
You can have a look at the attached diagram.
See a attachment.