As of this morning in Mesa Git, the compute shader support code in the NVC0 Gallium3D driver has been enabled for GeForce 400/500 "Fermi" graphics processors. This Fermi compute shader support code was previously sitting on the Mesa-dev list and goes along with the Gallium3D compute shader work already committed to mainline Mesa by Pitoiset.
As noted by Samuel in the commit message, "Kepler compute support is really different than Fermi and it's not ready yet." Enabling compute shaders in Fermi for the Nouveau NVC0 driver landed over the course of several patches that landed this morning.
Mesa 11.2 was supposed to be branched on Friday, but that didn't happen yet, so this latest Git activity will still end up in the 11.2 release.