Been helping a few people out with this issue and an irate Dane just convinced me it needs a Blog post.
Quite simply, the issue occurs when you install the Web Driver, do a reboot, and find that the OS X Default Driver is still selected. Even when you select the Web Driver again, on reboot you find it still is using the OS X Default.
I have seen this myself and don't know exactly WHAT is happening. Good news, I know how to fix it. And at this point be very glad you have an MVC EFI card, because4 those with unfleshed cards are staring at a black screen.
DO a good, old fashioned PRAM reset. Use the 4 key method or a terminal command, whichever you want. Point its, on next boot it will be on OS X Default again. This time when you select Nvidia Web Driver you will find it sticks.
Has worked in 100% of cases so far. Enjoy your cMP, new ones just a year or two away!