I have the same problem on my 3.1 with dual 8800GT.
The interesting thing is that I can have 2 out of the 4 monitors working in portrait mode, while the 3rd and 4th have the black screen (but do see my mouse curser).
First... I did an upgrade from 10.7 to 10.8 thinking that perhaps the video driver getting messed up, but same result with a clean 10.8 install.
That said... MP 3,1 with 10.8 is only working with 2 in portrait and 2 in landscape mode to use all 4 displays.
😡😡😡