From Read/Write Web:
Google just opened the Google Wave developer sandbox for federation. Developers can now begin prototyping tools against WaveSandbox.com. Google tested earlier versions of Wave with a small number of developers on the Wave sandbox and this server will now 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.