As much as nVidia would love to cut the number of driver revisions they need to write in half, I don't see them discontinuing support for XP any time soon. The overwhelming majority of gamers will stick with XP until they have a real reason to use Vista (DX10 games). Idk about actual numbers, but I'd expect something like 80% XP and 20% Vista in the next 3 months, with a 5-10% increase in Vista users per month after that.
What I would like to see (if they're not already taking this approach) is asynchronous updates for the different cards/OS platforms. Realistically, 7-series drivers for existing games in XP are pretty mature, so they require less updates than the 8-series drivers for XP and Vista along with the 7-series drivers for Vista. My biggest concern is nVidia holding off on driver updates for all 4 OS/GPUs until they have a revision for all of them, instead of simply releasing updates as they become available for any of the single OS/GPUs.