GLSL 1.40 is a requirement for OpenGL 3.1, which Intel is now attempting to support in Mesa now that they have OpenGL 3.0 / GLSL 1.30 support for their Intel Sandy Bridge and Ivy Bridge graphics driver. There's been talk of possibly having OpenGL 3.1 in place for Mesa 8.1, which will be released this summer, and GLSL 1.40 is part of this.
Eric published the first pieces of GLSL 1.40 support via the mailing list. He's also submitted GLSL 1.40 Piglit test cases.
Besides GLSL 1.40, other OpenGL 3.1 work left to be accomplished within Mesa is support for texture buffer objects (GL_ARB_texture_buffer_object) and uniform buffer objects (GL_ARB_uniform_buffer_object). Most of the other OpenGL 3.1 work has already been completed for the Intel driver as well as the R300/R600 Gallium3D drivers.
Of course, once OpenGL 3.1 support is complete there's still versions 3.2 and 3.3 and then OpenGL 4.0/4.1/4.2 until caught up with the latest Khronos specification, by which point there will likely be several new revisions.