i HAD the same, i was experimenting with my mobo´s integrated GPU, to dedicate it to physX, and i remember that i put physx and sli markers visible from nvidia CP, to see if it works, and when i first launched the game it started. I then rebooted and i think the reboot fixed it.
Goddamn i cant remember well what configuration i had in bios and nvidia CP.
Though i got it to work with integrated + 2x9600GT in sli.
I think it might be driver issue. Clean all tracks of your drivers, uninstall them properly, clear all GPU´s from your device manager, and install them again. It might work. Check your sli-marker status from nvidia CP and so on.