Google just opened the Google Wave developer sandbox for federation. Developers can now being prototyping tools against WaveSandbox.com. Google tested earlier versions of Wave with a small number of developers on the Wave sandbox and this this server will become the platform for testing interoperability between different Wave servers. Google also released a how-to document that explains how to set up a Java-based Wave server over the weekend. More details about how to implement the Wave Federation Protocol can be found here.
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Setting Up a Wave Federation Prototype Server
Developers who want to run their own Wave server and find all the information for setting up a Wave Federation Prototype Server in these documents. The server is written in Java and should run on Windows, OSX and Linux-based machines. In the documentation, the Wave team explains how to run the server as an extension to the open source OpenFire XMPP server, though any XEP-00114 compatible XMPP server should be able to talk to the Wave server.