The author also notes that while he’s not sure which license he’ll utilize, there’ll be at least some degree of documentation:
One thing that you’ll find for sure, though, is a paper describing all of the techniques used. Including the portals and the rest of the physics, and some of the rendering (lighting through portals which is planned, etc.). There’s also tons of Lua to be finished (right now just the portals and “some” physics are Lua-scriptable) which is “open source”, of course, and lots of shaders which I’m planning on releasing as “learning resources”, especially for Ogre3D users.
It’s not available yet, but there will be a version 0.5 public beta with Linux support.