Well...I'm no great technical expert on Poser but from what I understand, the lighting models were changed pretty drastically in Poser 6 from 5 (I went from 4 to 6, so I can't verify this myself). Along with the "no light" effect you mentioned, I've noticed that when doing a render with "ignore shader trees" checked, I'll sometimes also get some weird background and floor shading effects (i.e: shaded when they shouldn't have been).