The OpenGL/Vulkan implementation is not open-sourced yet, but it looks like the core driver code is.
I remember reading that Nvidia shares most of its driver code between OSes (e.g. FreeBSD and Linux drivers are extremely similar). So if/when the OpenGL/Vulkan part is open-sourced, 3D accelerated Nvidia drivers for Haiku become a real possibility. Should be much easier than porting Linux AMD drivers.