Earlier quoted context omitted.
AFAIK both Windows (WDDM) and Linux (DRI) use userspace drivers. http://en.wikipedia.org/wiki/Windows_Display_Driver_Model
You're definitely correct about Windows (it was something I was excited about in Vista), but I'm not sure about Linux. Looking at the Nvidia GLX drivers for my Linux machine, it still seems like quite a hefty kernel module. I can't really examine it, though, because it's not open source.
The open source drivers also follow this kind of model where the kernel modules handle hardware access and memory management but not a whole lot more. Mesa (and Gallium 3D) all work in userspace to handle compiling things to the native formats of the cards.