NVidia's latest SDK (v4.2.9) does not support OpenCL 1.2 with regard to the header files or library it provides. I considered this might just be the SDK itself: as you point out, the GeForce 600 series appears to support it in hardware. Unfortunately I don't own any 600 series card, but OpenCL64.dll supplied with the latest drivers (v306.23) does not export OpenCL 1.2 symbols. Further, I can find no trace of the new symbols (such as "clLinkProgram") as strings in the driver package. Although this does not rule out the possibility of bootstrapping 1.2 functionality in the driver via an ICD Loader, there is no evidence that there is an 1.2 implementation there, and this would be undocumented and unsupported.
As to when OpenCL 1.2 will be officially supported by NVidia, unfortunately I don't know the answer to this, and would be equally keen to find out.
In the mean-time you might consider an alternative OpenCL 1.2 implementation for development; for example the Intel SDK 2013 Beta (Intel CPU) or AMD APP SDK v2.7 (AMD CPU or AMD/ATI GPU).
An aside, but personally I am considering switching from NVidia GPUs to ATI for production purposes, partly based on AMD's investment in OpenCL and also arguments comparing "bang for buck" between NVidia and the latest AMD cards: NVIDIA vs AMD: GPGPU performance
