Update: Good Vibrations by Radovan Semančík. Lot's of interesting questions about how Wave works, scalability, security, RESTyness, and so on.
Google Wave is a new communication and collaboration platform based on hosted XML documents (called waves) supporting concurrent modifications and low-latency updates. This platform enables people to communicate and work together in new, convenient and effective ways. We will offer these benefits to users of Google Wave and we also want to share them with everyone else by making waves an open platform that everybody can share. We welcome others to run wave servers and become wave providers, for themselves or as services for their users, and to "federate" waves, that is, to share waves with each other and with Google Wave. In this way users from different wave providers can communicate and collaborate using shared waves. We are introducing the Google Wave Federation Protocol for federating waves between wave providers on the Internet.
Here are the initial white papers that are available to complement the Google Wave Federation Protocol:
The Google Wave APIs are documented here.