Jonas Sundstrom pointed at a open source NVIDIA driver that supports 3D, and currently runs on BeOS. Could it work under X? Maybe, with a pile of work.
Utah-GLX (http://utah-glx.sourceforge.net) had working Geforce 3d support years ago. At least for NV1x based solutions (GF1, 2, 4MX) that’d be the best route for somebody looking to write a driver to take.
DragonFly BSD Digest