A new experimental report has now improved upon the validity of the isotropy of the speed of light, as reported on PhysicsWeb (link requires a free registration).
Simultaneous tests of Einstein’s special theory of relativity performed in Europe and Australia have allowed physicists to conclude that the speed of light is constant in all directions, without having to make an important assumption that had limited the validity of previous tests. The researchers performed two different types of Michelson-Morley experiment -- one on each continent -- which allowed them to rule out, for the first time, violations of Einstein’s theory that could affect the physical properties of the experimental equipment and change the outcome of the measurement.
The preprint for this work can be found on ArXiv here. They have successfully ruled out the influence of the changes in the apparatus as the possible source of the isotropy in the speed of light.