I think the issue might be that the client (ServiceSoapStub, etc.) you are using does not know how to talk Kerberos.
I'm not certain but there's probably a way to do it if you search through the websphere docs for words like single-sign on, spnego, wsdl, soap client stub, etc.
There is an open source project on sourceforge that might be of some interest. The project is meant to be a server side project but the library has a stand alone client piece that can be used against any server.