There's a new .NET reference application out from Microsoft that showcases both interoperability with the Java platform and scalability on the .NET platform. The application is called .NET Stock Trader and it was built by Greg Leake and his team at Microsoft. I had consulted Greg for information back when I was working on the .NET/Java interoperability guide for the patterns & practices team. At the time, he had just completed work on one of the few iterations of the .NET Pet Shop application, a take on the Java Pet Store application from Sun.
The .NET Stock Trader application is an end-to-end sample application for .NET Enterprise Application Server technologies. It is a service-oriented application based on Windows Communication Foundation (.NET 3.0) and ASP.NET, and illustrates many of the .NET enterprise development technologies for building highly scalable, rich "enterprise-connected" applications. It is designed as a benchmark kit to illustrate alternative technologies within .NET and their relative performance. The application offers full interoperability with J2EE and IBM WebSphere's Trade 6.1 sample application. As such, the application offers an excellent opportunity for developers to learn about .NET and building interoperable, service-oriented applications.
This looks like a great example of interoperability between .NET and Java in an enterprise scale application. You can get the bits and read the associated white-papers on MSDN here.