This proposal is being considered so that all old user-space compatibility is gone, there is no more user-space mode-setting to support (the code is already removed), and so that there can finally be an officially released version that Linux distributions can utilize and support. Though at the same time, the 3D driver with the classic Mesa driver for the very first NVIDIA GPUs and the Gallium3D drivers for all the other GeForce graphics cards are not complete. Without the 3D drivers being "done", there is the possibility that the API still may need to change for them.
David's proposal can be read here. Stephane Marchesin who is the founder of the Nouveau project has already responded saying the Nouveau developers really need to focus on making the driver perform faster tuning and there is still concern over API breaks.