Precise vBlank timing support has been patched for the Nouveau DRM driver to support the NV50 through NVC0 NVIDIA GPUs (up to and through Fermi but not yet any NVD0 or Kepler hardware).
This support was written by Canonical's Maarten Lankhorst and has now surfaced on the dri-devel mailing list
. This is a potential candidate for merging into the Linux 3.12 kernel.
For details on precise vBlank timestamps for Linux along with background information as to why this is important, see the earlier Phoronix article on Intel, Radeon DRM Get Precise VBlank Timestamps