A few months ago, I started thinking about an implementation of extending Sparse Voxel Octree raycasting with realtime GPU accelerated path tracing as a proof of concept. Well, no need for that anymore as the guys behind Voxelstein 3D have beaten me to it.
Hans Krieg, the (only?) programmer of the Voxelstein 3d engine incorporated a path tracing extension to the engine which uses SVO and BIH (bounding interval hierarchy) to render the voxels. The code runs on both CPU and GPU(CUDA) and the first images are rather crude, but have a warm and natural global illumination to them.
For pictures and details about the engine, visit http://www.jonof.id.au/forum/index.php?topic=1411.msg12406#msg12406
UPDATE: Samuli Laine has written an excellent paper entitled "Efficient Sparse Voxel Octrees" which uses a nifty trick to make the contours of the voxels much less blocky. On top of that, he posted a video and the complete open sourced CUDA code of his technique as well. All can be found, read, watched on http://code.google.com/p/efficient-sparse-voxel-octrees/ . Fantastic work and probably the most practical and "efficient" implementation of SVOs yet, ready to be abused by graphics engineers :-)