I have written up a document that is an overview of the goals and rationale behind DefDoc. It is available as LaTeX, PDF, and Postscript.
The code is very incomplete, but what there is is publically available. There are definitions for various types of document elements. There are also definitions for the abstract types of layout engines as well as the HTML layout engine. Finally, there is a basic frontend, essentially a few simple macros for creating DefDoc documents. A more fully-featured frontend is planned, which will be sensitive to the current context and allow texual content to be entered unquoted.
This site is actually a DefDoc document.
There are 3 mailing lists for DefDoc, defdoc-announce, defdoc-devel, and defdoc-cvs.