Just stumbled upon this upcoming paper about interactive GPU path tracing from Niklas Henrich. The abstract sounds very promising:
"Recently, path tracing has gained interest for real-time global illumination since it allows to simulate an unbiased result of the rendering equation. However, path tracing is still too slow for real-time applications and shows noise when displayed at interactive frame rates. The radiance of a pixel is computed by tracing a path, starting from the eye and connecting each point on the path with the light source. While conventional path tracing uses the information of a path for a single pixel only, we demonstrate how to distribute the intermediate results along the path to other pixels in the image. We show that this reprojection of an eye path can be implemented efficiently on graphics hardware with only a small overhead. This results in an overall improvement of the whole image since the number of paths per pixel increases. The method is especially useful for many indirections, which is often circumvented to save computation time. Furthermore, instead of improving the quality of the rendering, our method is able to increase the rendering speed by reusing the reprojected paths instead of tracing new paths while maintaining the same quality."
Hopefully, the results from "Accelerating path tracing by re-using paths" (a paper from 2002 by Bekaert et al. who reported a 9x speed-up for CPU path tracing) can be replicated on the GPU.