Ok after reading a lot of stories, here is mine :
i have a late 2008 unibody macbook pro 2.53Ghz with both 9400 and 9600 512mb VRAM.
i noticed 1 night that at the last low level light setting, my screen brightness would wave a little bit. Then i got worried and follow the instruction of reseting PR RAM. (under OSX 10.6.2) and it worked.
Strangely, the problem is gone under OSX, doesnt matter which card i select, the problem is hopefully gone forever.
Now, under windows 7 ultimate, i meet the same problem. At the last level, my screen brightness would wave intermitently, which is kind of annoying.
i tried to figure out what the problem was as i had Bootcamp version 3.1 installed :
1) i downgraded my bootcamp drivers to 2.1 (problem was half solved)
2) i change my 9600m gt graphic drivers with laptopvideo2go optimized video drivers (188.25 one of the best drivers) and problem was still here
3) i finally came to the conclusion that its a software issue cause Apple didnt give us yet a new bootcamp update that would make both cards recognize by Windows.
At the moment, only the 9600m gt is visible by the system, but not the 9400.
Conclusion : fear not my friends, we may see a spectacular update in the next version of bootcamp or OSX that will get rid of this problem.
by the way, i have the 09c84 screen, use windows 7 X32 Ultimate, OSX 10.6.2