I think the crossover point I mentioned earlier will come when GPUs become general-purpose enough to allow massively parallel voxel rendering implementations I

I think the crossover point I mentioned earlier will come when GPUs become general-purpose enough to allow massively parallel voxel rendering implementations Indeed. This is certainly possible with modern GPUs, and has been for a while. Voxels are pretty well-suited to GPU rendering. See the research I linked to in another reply in this thread. I didn't get around to reading their entire article, but if they call it a CPU-only technique they are behind the curve instead of ahead.