Outcast (game) was released 15 years ago (1999), and uses a heavy mix of early ray tracing with their height-mapped based terrain being treated by the rays as voxels, and it ran fully in software.

Raytracing is a very interesting technology, and developers have been integrating it into their polygon-based engines (with octrees also) for many years, doing it more and more as the hardware allows.

Also, every year or so someone will bring up a technology where the company, such as Euclideon, keeps claiming it will allow "infinite detail" or "unlimited detail" using voxel-based rendering.

These are basically precomputed, voxel-based octrees that were all the rage in the 1970s. Storage speed and transmission speed have both increased, but still it is only mildly useful in games. There were many different algorithms in the '70s and '80s over it. Marching Cubes, a moderately efficient voxel isosurface algorithm, was released and patented in 1987. The patent hurt the research field rather painfully until it expired in 2005.

A very relevant video:

For how long has voxel-based ray tracing been on the verge of revolutionizing gaming graphics and subplanting the triangle as the prefered rendering primitive? 10 years? 20? I remember people talking about the imminent pending death of the triangle back before fixed-function GPUs turned into programmable GPUs, and it was an old argument even back then.

Delta Force, anyone: http://en.wikipedia.org/wiki/Delta_Force_%28video_game%29#Technical_features ?

Or Comanche? (1992): http://en.wikipedia.org/wiki/Comanche_series :)

nVidia is finally putting Crassin's work to use with there voxel power GI (VXGI), but other than this, it seems the trend is to supplement conventional polygonal rendering with certain voxel-based techniques.

Voxels will never be as practical as using a surface-based representation once the scenes get to be large. The number of voxels is generally cubic in the scene size, whereas triangles can have arbitrary level of detail. It just takes too much memory. Imagine ray tracing a voxel scene that is 1km^2 with a voxel resolution of 10cm. I would rather use a small BVH (10s of MB) than have GB of voxel data. Voxel traversal is popular because it's easy on the GPU, but doesn't scale well.