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Interoperability has been a primary goal of the web services standards from the beginning. After all, why would you go to the trouble to form a cross-industry initiative to essentially re-invent advanced distributed programming protocols like CORBA and DCOM, unless you could create standards that were broadly supported and truly interoperable? Interoperability is also fundamental to the whole promise of service-orientation. From it's inception, WCF was created to enable this interoperable, service-oriented vision.
Achieving something of this scale on an industry-wide scale is not an easy task. It's been a long road, but Microsoft and the other platform vendors have stuck with the vision of SOAP and WS-* and the end result is solid and getting better with each release of the various platforms that support it.
We've just published a new page on MSDN to be the one place you can go to learn about web services interop. We have information there about current interop test results between WCF in .NET 4 and the latest releases of the major Java web services stacks. We also have white papers that show specific scenarios that are supported and How-To guidance for developers. And we have information about the various interop activities and communities Microsoft is involved in. Check it out.