As talked about on the GTX 680 launch day, two members of the Nouveau community managed to get access to early Kepler hardware through unspecified means. At launch they already had the support working for the Nouveau DRM/KMS driver so it could be pulled quickly into the Linux 3.4 kernel, which had its merge window open at the time. Afterwards they pushed the support into the xf86-video-nouveau DDX and now they have support for Kepler in Nouveau Gallium3D. Benefiting them is that NVIDIA's graphics architecture didn't change too much in going from Fermi to Kepler.
As said in this commit today, "Most things that work on Fermi should work on Kepler too. There are a few performance optimizations left to do, like better placement of texture barriers and adding scheduling data to the shader instructions (without them, a thread group will be masked for 32 cycles after each single instruction issue)."
The changes between NVIDIA's GeForce 400/500 "Fermi" series and the GeForce 600 "Kepler" isn't as nearly as invasive as going from the Radeon HD 6000 "Northern Islands" / "Cayman" to Radeon HD 7000 "Southern Islands" with the new Graphics Core Next (GCN) architecture, but still the Nouveau driver has managed quite nice timing in this case. For reference, the Radeon HD 6000 series (which wasn't as invasive in going from Northern Islands to Southern Islands / GCN) took two months before the support arrived to succeed AMD's Radeon "Evergreen" generation. Adding in the Radeon HD 6900 Cayman support also took some time.
Meanwhile the Nouveau developers reverse-engineered and tacked in support for the latest-generation hardware in less than one month. This isn't to bash the small open-source AMD crew, but to say nice job to the Nouveau developers. Enabling the NVIDIA GeForce GTX 600 series open-source support in the Mesa repository just resulted in a net of around 600 lines of new code.
As usual though, an initial limitation of this new-found open-source support is that the Kepler acceleration code requires the external NVIDIA firmware to be loaded. This firmware isn't re-distributable so first you must load the NVIDIA graphics driver while MMIOtrace support is found within the kernel and then dump the bits. After that point, the Nouveau Kepler support should hopefully be good to go. I need to send back my GTX 680 Kepler soon, but will try to run some quick Nouveau benchmarks versus the NVIDIA binary blob on it this weekend.
The Nouveau Gallium3D Kepler support has evidently been done for a few days already but was held up by the massive Nouveau code push with the new libdrm interface.