There is a guy called Branislav Siles who is working on a cross platform voxel system that is more than good enough for AAA games.
I have been trying to get more details from him for ages, but the videos are awesome. Much nicer than unreal for a start.
The basic technique I have got out of him is he uses a very efficient sparse 3d array to store mipmaps of voxel objects.
So if you are a long way away from an object it is displayed as a quad, get a bit closer and it's displayed as a few quads , ad nauseum
The voxels are displayed as billboards, so each one is very quick to render.
The clever bit, is the storage of the voxels and the way he parses it in the game. He's not talking about that.
There are other demos of efficient sparse 3d array rendering on gpu's which are just beautiful.
I disagree with Reedbeta on this one, voxels are the way forward, just not voxels as we used to think of them. :rolleyes:
You could also research things like "voxel polygons" which is a nice way of getting around a few of the issues with voxels.