I think I remember Doom 3 actually using displacement maps in real-time, but that's the only direct application of it in an engine I've seen. However, now with DirectX 11, it can be used for further real-time tessellation.
the stone floor image above is not a displacement map... there is no hight changing in the geo... as you can see at the border of the plane... thats a parallax map... http://en.wikipedia.org/wiki/Parallax_mapping displacement maps are pushing verticies along there normal...…