It's been years and years since I knew the details, but at one point the nvidia drivers replaced pretty everything from libX11 down, and I don't think you could hook in at a middle point. (ie, they don't support the "standard" graphics stack.) Would appreciate if anyone could comment on how things stand today.
Somewhat better, but still awful. They don't replace libX11, but they do replace libGL, and they bypass the entire kernel graphics stack in favor of their own.
