Making some progress on this, however....
I've logged in and gotten my sessionID, etc. from the salesforce.com WS but subsequent calls require that the server URL be set to the value returned with the login request.
How can I do that? I've set these all up in advance to the be main entry point for the WS?
This is exactly the same problem I face when I want to change a REST url call (Technorati, etc.).