This is one of the coolest things I've seen all year, without exaggeration. The gradual rendering alone is new to me (and utterly fascinating), but the fact that diffuse lighting can be rendered in a browser... this completely redefines what I understand a browser to be capable of. Seriously amazing.
If you haven't seen path-tracing before, you should check out LuxRender: http://www.luxrender.net/ It's an open source unbiased renderer that uses similar techniques.
It's a massively parallel problem, so the recent abilities to run general purpose code on a graphics accelerator has really been a huge boost to performance something like: http://www.youtube.com/watch?v=70uNjjplYzA would have been impossible before CUDA came out.
