The Nouveau "NVC0" Gallium3D driver was outright broken for Nexuiz while it continues to work with the "NV50" driver.
OpenArena had run on the Nouveau Fermi stack, but its performance was very poor for the GeForce GTX 460. The proprietary NVIDIA driver was around fifteen times faster. The driver reported to be running at the stock (675/1800MHz) frequencies like the proprietary driver, but this may have been incorrect based upon the vast performance differences or other serious shortcomings in the open-source driver.
Regardless of the driver problem's cause, the "out of the box" Nouveau Fermi performance on the latest kernel and Mesa is quite poor. With the Warsow game, it was around a 14x difference.
Lastly, with both drivers in an "out of the box" configuration, the NVIDIA 280.04 beta driver was around 30 times faster than Nouveau Gallium3D on the Linux 3.1 kernel with the Lightsmark OpenGL benchmark. These results can be found on OpenBenchmarking.org.