As others have said looks like some sort of voxel raycaster. I wrote one a couple years back that got to about 1FPS w/o too much optimization so it's pretty conceivable that with better optimization and faster hardware you could go much faster. The big problem with building scenes like this is that the datasets get really big really fast. There's a lot of interesting work happening in the area, in particular id's upcoming stuff and the GigaVoxels project.

http://www.youtube.com/watch?v=HScYuRhgEJw http://www.youtube.com/watch?v=VpEpAFGplnI http://artis.imag.fr/Publications/2009/CNLE09/