Igalia's Iago Toral provided an update on the FP64 support today that they believe the work is "now mostly complete, in the last days we have been going through the patch set cleaning things up a bit in preparation for review...It still needs a rebase and some clean up work, but we should be sending this for review soon too. As it is now, we pass all the fp64 tests, except for a few that fail because of spilling, that could probably take some optimization work to fix. We also have no regressions in non-double functionality in any hardware generation that we have available (gen5 to gen9)." Those details were shared via this FreeDesktop.org bug comment.
While the work appears ready, it could take quite some time to review: FP64 support for the i965 classic driver is around 190 patches! Nevertheless, the next major Mesa release (11.3) is three months out so there is time to get it completed for the next milestone.
The ARB_gpu_shader_fp64 support has been the lone extension blocking Intel's Mesa driver from OpenGL 4.0 compliance for a number of months. The Intel driver's only blocker for OpenGL 4.1 in turn is the related ARB_vertex_attrib_64bit. Intel's Mesa driver already has all the OpenGL 4.2 changes complete. Thus it's looking like for the next Mesa release there will be Intel OpenGL 4.2 support! That is, if you're using recent Intel HD/Iris graphics hardware.
Blocking the Radeon and Nouveau drivers from GL4.2 remains just ARB_shader_image_load_store and ARB_shader_atomic_counters (with the latter extension extension already being done for Nouveau NVC0).
With OpenGL 4.x advancements ongoing for Mesa and Intel concurrently working on their Mesa-based Vulkan open-source driver, 2016 should continue to be another interesting year for Linux graphics.