Interoperability means many things to many people. I have talked to IT Professionals and software Architects around the world and there are two concepts that consistently bubble up when we discuss interoperability: Connections and Collaboration. Connections are the conduits that exist between people, data, and systems. Collaboration occurs at two points: up-front collaboration of various kinds to enable these connections, and once the connections are created, the collaboration that results from this interoperability.
The recent completion of the Windows Live Messenger and Yahoo! Messenger bridge (http://www.betanews.com/article/Microsoft_Yahoo_Bridge_IM_Networks/1152759304) is a perfect example of collaborative interoperability. There is no prevailing open standard for real-time IM communication - both Microsoft and Yahoo! have created their own services and protocols to meet the needs of their customers. Without a formal standards body, each was able to agree to a common protocol for basic inter-service communication with a promise to increase the feature set supported by this protocol in the future (including voice etc.).