Duh! I should completly read your first post before responding! Well that is exciting then. Increasing the photon count will probably help - but it may start driving the render time up longer than you'd like. However, there are two other things that I would try: 1) select the geometry in the vertex modeler (you'll need to convert it first) and crease the whole object to make sure Carrara understands that it's a completly flat surface, or 2) place a cube behind the far wall and allow it to intersect the wall slightly to prevent any "light creep" that may be occuring at the corner.