In fact, some more news on that. I just got an email back from the Senior Acquisitions Editor at Course and they are extremely interested in my book. I am just translating my own proposal into their own format before they formally accept it and send a contract my way.
On another note, I fixed the issues with the binary serialization in Nexus World Builder. Aside from bug fixes I cleaned up the design quite a bit by employing a .NET savvy visitor pattern. New format is quite flexible and clean.