We keep developer documentation, articles, etc. at the Mozilla Developer Center.
Probably the best way to learn how to use these technologies is through examples. There is a nice introduction with examples in the spec.
There is a pretty active mailing list available which carries all kinds of news of the XForms world. This is also a great place to post questions and concerns about authoring XForms documents.
Other examples are available from the various XForms implementations. We will endeavor to add test cases to a test directory as we progress. We'll link to our test directory from this project page once we have our place in the tree set up. We'll also link to our Mozilla-specific documentation as that becomes available.