Roy has published a set of patches that improve the Nouveau clock tree parsing and from hsi tests he's now found the engine re-clocking to be stable on the hardware he's tested, but the memory re-clocking is still up in the air.
This patch series is for the NVIDIA "NVA3" GPUs, which in marketing names come down to the GeForce series GT 240, GT 320, GT 335M, GTS 250M, GTS 260M, GTS 350M, and GTS 360M. The NVA3 workstation part is NVIDIA's Quadro FX 1800M. For the most part these are pre-Fermi GeForce 200/300 series mobile GPUs.
Roy's patch set can be found on the Nouveau mailing list and will hopefully be ready for merging into the Linux 3.18 kernel -- hopefully with re-clocking being enabled by default for these select GPUs on this open-source NVIDIA driver.