Generally when a new operating system is launched it takes time for hardware manufacturers to catch up. They will oftern launch with a good enough driver whilst the developers re-learn everything in order to get the most out of the hardware on that OS.
The trouble with Vista is it has moved the goal posts so many times the manufacturers are sometimes not even sure what game they are playing. Why write drivers for an interface that is not set in stone and you have to rewrite next week? How do you work with DRM?
A lot of corporate users consider Vista a distraction and are still rolling out XP. If nVidia wish to ignore corporate and governments that is there choice but not many other people buy PC in units of 10k plus 😉
No. I seriously doubt that nVidia would drop XP support but it might get massively distracted trying to 'FIX' problems caused by underlying Vista issues.
Just my thoughts