I had a brilliant idea last night. No details will be made public yet, but I think I've worked out a way to make photon map-based global illumination almost independent of the size of the geometry database. At any rate it should bring memory usage down to the point where you could do a large frame over a small number of render nodes at the cost of a small amount of help from the modeller.
Unfortunately, it probably won't work for caustics (or at least it won't work as well), and it's completely incompatible with the Renderman interface (so my REYES stuff isn't any help here). Still, this is exciting stuff and I can't wait to start implementing it. Now all I need is a ray tracer to build this on top of...
Fortunately, I don't work for anyone working in visual effects any more, so the IP is all mine. Mine, I tell you!