The point of WS is not to talk to non-Java services, the point is that it doesn't matter which language/technology a service is implemented in. Java clients often talk to Java WS without even knowing that the same language is used on both sides. SOAP and WSDL abstract this away.
I can't imagine the SCJA going into any depth on this, though.
Web services is a great means for components to communicate over the web. Communication happens with XML messages, which is not specific to any particular platform - Java, .Net, CORBA, C++, etc. As long as XML is understood, different components, written in any range of languages, can communicate. That's the beauty of web services.
For the SCJA exam, you should know the technologies that make web services work, namely, know the basic purpose of WSDL, UDDI and SOAP. I've heard of high level questions on each of those technologies.