Here are 30 renders of a simple little setup from various angles. This is with an ordinary point light, located at the position indicated by the little white ball. The light is not generating any specular component as I set that to black. I also set the shader on the box so that there was no specular. So we're only seeing the results of the diffuse calculation built into the Poser root node.

Some key things to observe:

Most of the ceiling is dark - clearly the influence of L dot N is at work here. Jon - this supports your results where your ceiling was not lit very well, if at all. You're not going to be able to light up the ceiling, accept where the alpha is very small. We can see that in your render just fine.

Look at the blue arrow. Compare the wall's brightness to that of the render marked by the white arrow. Those arrows are pointing to the same spot, yet the white arrow one is clearly much darker. The only difference is viewing angle. This is why I suggest there is another term we don't know about. By the way - this issue of viewing angle has troubled me from the first time I used poser. Walls always look wrong to me, because in real life (as well as the definition of Lambertain reflectance) there is no difference between one viewing angle and another. My office walls look the same brightness no matter where I stand.

Finally I've discovered a bug in Poser. Observe the yellow and red arrows. There is an abupt increase in brightness despited only a tiny change in viewing angle. This is most disconcerting when viewing these images as an animation. In a still, you wouldn't notice it, but in animation it is incredibly obvious. It looks like the light suddenly got stronger. This is a bad bug.