As part of open-sourcing Wave, we are documenting the various aspects of the Wave protocol and making those documents available here. For any questions on the documents, please post in the Google Wave Protocol group.
The design docs page is an index of implemented designs and proposals for significant changes and improvements to the Google Wave protocols and models.
Some implemented designs:
These were published in May of 2009, and parts may be outdated or not in line with the current protocol.