SandyJS, Nice render, looks like you got it all working!  Populus has some interesting notes which I hadn't explored.  I don't clearly know what the photon map accuracy does.  My solution has been to jack up the photon count.  The Cornell Box used 100,000 count with accuracy set to 100%.  It was the only way to get the corners to work out.

GKDantas, yes, the Cornell Box was rendered in Carrara.  As you can see, a lot of the lighting physics are computed (the green scattered onto the floor, the lensing through the ball, etc..  The only light source is a single square panel in the hole in the ceiling with a glow set to white.

-Kix