Saw this on Blendernation.
Go here for latest version + docs:
http://jerome.le.chat.free.fr/3d/cityengine/
For discussion on development:
http://blenderartists.org/forum/showthread.php?t=140415
I’m not sure how usable this will be if imported into Unity. Plan on testing it out later today!
Anyways, could be good for prototyping, or a place to start.