I have spent the last weeks on my idea of little game with real-time path tracing named Sfera. It is mostly written with OpenCL and uses Bullet Physics as physic engine. The source code and some pre-compiled binary is available at http://code.google.com/p/sfera
A demo video is available at http://www.youtube.com/watch?v=Dh9uWYaiP3s
and should explain how Sfera works.
Sfera includes few un-conventional technical choose like a BVH with sphere bounding box (to reduce GPU threads divergence), brute force path tracing (i.e. no direct light sampling), the capability to store many data on constant GPU memory, etc. As result it can achieve some impressive number in term of raw samples/sec:
1,115,340,000 samples/sec is a pretty big number, no matter how you turn it
Sfera is heavily based on SLG2 code (for instance the level description language looks very much like a SLG scene).