I first read about the concept of mixing Unity and OpenSim through a post by epredator on his Life at the Feeding Edge, and always wondered if such an experiment would ever become reality.
Rezzable recently announced that they had successfully achieved a prototype of Unity3D and OpenSim working together.
Several months ago, I had visited this same installation through an OpenSim viewer, but I was now able to visit the same installation (built with prims in OpenSim) but viewed from the web through the Unity web player.
This is very exciting news, and I suspect we will see more activity around connecting Unity3D and OpenSim in the months ahead!