Google announced the developer instance of Google Wave for experimental interoperability testing with other wave providers. This means that if you’re interested in building a service that uses Google Wave Federation Protocol, you can begin prototyping with a tool like FedOne against WaveSandbox.com. If you'd like to dive in and get started, check out introductory docs. You can learn more by reading detailed status note on wave protocol forum. You can also contribute to development of growing open source reference implementation; check this guide to submit open source code.