Kerkythea, which I mentioned earlieris another freepath to Metropolis Light Trasnport type physical rendering.

"I've never understood why they picked something like firefly for poser." 

Daz went the same route with 3Delight though the underlying algorithms may be different between Renderman compliant renderers as long as they implement certain capabilities and meet the API if I understand it correctly. I imagine cost and ease of integration played some role in the decision. Given that they've been exporting RIB since P4 at least, it may have minimized the coding but that's a total guess. Max has MentalRay built in but most of the Max scenes I see seem to use VRay so apparently no one is content with what comes in the box :-)

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken