The backend is coded in Symfony (PHP Framework). For the 3D client I will be using Unity 3D.
@James: I've thought about it before. I just don't know yet how to expose it nicely. A web-based wizard would be cool, a "Quest Builder", and it's not horribly complicated, just a little.
The difficulty is that I have a few tricks that I would need to explain and document nicely, especially how to build branching quests or alternate endings.