There is zero incentive for nvidia to write the driver, nvidia writes drivers to drive hardware sale. But in the case of mac the fact that a nvidia card sits in it is not the driving force for the sale. But lets say that they would be so inclined to write the driver and take up support for it, were would the talent to do so come from. I don't really see nvidia brimming with top notch software people that can code for the mac at the moment.
Having said that, i'm sure that apple consults with nvidia when creating their drivers for it.