GL_ARB_shader_image_load_store allows GLSL built-in functions for shaders to load, store, and atomic read-modify-write operations on a single level of a texture object (images) during any shader stage. The OpenGL extension is documented at length within the specification found at OpenGL.org. This extension is required for full compliance with the OpenGL 4.2 specification.
Francisco has been working for some time on this GL extension for Mesa/Gallium3D and while it's still considered a work-in-progress, the initial low-level code for the extension was pushed to Git master today. The committed work has been to Mesa's common area and not specific to any driver.