OpenCL 1.2 is backwards compatible with OpenCL 1.1 and 1.0 while new to the public specification is support for seamlessly sharing of media and surfaces with Microsoft DirectX 9/11 APIs, enhanced impage support, custom devices and kernels, device partitioning, and separate compilation and linking of objects. The OpenCL 1.2 specification update isn't as large as going from OpenCL 1.0 to 1.1, but is still significant.
The complete OpenCL 1.2 specification can be viewed on the Khronos Registry. There's also the OpenCL 1.2 quick reference card (PDF).
Additional details from this spec update during the SC11 super-computing conference can be found in the Khronos update.
The binary graphics drivers from AMD and NVIDIA will soon support OpenCL 1.2. However, sadly, the open-source support is still likely a ways off. There's been good progress with Clover for bringing OpenCL support over the Gallium3D architecture, but that's still a ways off and isn't even mainline yet. There needs to be OpenCL 1.0 support, then 1.1, and finally 1.2 support, but chances are OpenCL 1.3 will be out before the open-source side catches up. It's just like the game played on the OpenGL side with Mesa/Gallium3D still working on OpenGL 3.0 (a specification that's nearly three years old) while the latest Khronos specification and what's done by the binary drivers is OpenGL 4.2.