While there is the support there, it isn't fully complete and polished. The Nouveau Kepler support is just like the handling of Fermi and other recent NVIDIA GPU generations on this open-source driver where the OpenGL compliance is currently at GL 3.0, there isn't proper re-clocking support, there isn't any VDPAU support with PureVideo, the performance is much worse than the NVIDIA binary driver, and a number of other limitations that will not be addressed in the immediate future.
In this article are benchmarks from our Intel Core i7 3770K test system when using a NVIDIA GeForce GTX 680 2GB graphics card on both the open and closed-source graphics drivers. The tested Nouveau driver stack was a Linux 3.8 kernel development snapshot, Mesa 9.1-devel git-2f225f6, and xf86-video-nouveau 1.0.4. The binary driver used was the NVIDIA 313.09 Beta Linux release. All benchmarking was done from an Ubuntu 13.04 snapshot to compare the performance of the two NVIDIA Linux graphics drivers.