If you get the blank, black screen, and you switch to another console and restart KDM/GDM, all compiz.real processes get killed EXEPT the one from the user belonging to group "video". You can still kill the process manually, or you can provoke another blank screen in the described way. If you then restart KDM and do not have killed the renegade compiz.real process from the first round, you will have then two. You can repeat this procedure n-times and it will give you n-comiz.real's.
Another important thing: If you get the blank, black screen and you click with the mouse (no matter where), the mouse and the keyboard get immediately freezed. You cannot change to another user/console via strg+alt+F1 - 9. It will not work. But you can hit alt+print+b, e, i, s and f for direct kommunication with the kernel, wich still works.
Also, in very seldom occasions, when you start compiz with a user belonging to "video", X crashes at once leaving this entry in the logs:
I am using the nvidia96xx pakage from Lenny.