Since there's a lot less demand for it, drivers aren't updated at the same rate - not sure how it works on the windows side of things, though I imagine Nvidia and Ati are exclusively responsible for driver maintenance and push out updates regularly to give a "value added" proposition for their products. For Mac versions, Apple writes all the drivers for Nvidia cards (and manufactures them too, or at least puts Nvidia's parts together) and collaborates with Ati on their drivers. So the drivers are only updated when there's a specific need or OS update regularly scheduled - when Apple feels like it. Apple's never paid a whole lot of attention to GPUs, though I have hopes that this will change with their OpenCL initiative and the movement towards GPGPU computing.