Interoperability test are to make sure that the Web Service implementation of different type (Apache Axis, .NET, WASP, GSOAP, Axis C++) can be work with each other. The WS-I profile provide the guide lines and the there are tests that make sure that the interoperability requirements are met.
There are few of such tests
Round one tests, http://www.xmethods.net/ilab/
Round two tests, http://www.whitemesa.com/interop.htm
They are in different levels Simple types, Complext types … The tests provides the SOAPMessages and/or WSDL that explains the Web Services and the each SOAP implementation is suppose to write Servers that accepts the SOAP Message from all the client implementations and write clients that can talk to all other server implementations.
The task is to write a Server and Client side for the interop WSDL. The guide lines can be found from the Axis1.x interop tests (http://cvs.apache.org/viewcvs.cgi/ws-axis/java/test/wsdl/ interop, interop3, interop4 ...) and the Axis2 interop sample. (https://svn.apache.org/repos/asf/webservices/axis/trunk/java/modules/samples/src)