You wouldn't think that there's any reason to continue testing one of the most widely-used theory in physics, would you? But people do still test F=ma, and this time, it has been tested up to 5 x 10^-14 m/s^2. Take that! It appears that this came out of a similar technique from the University of Washington that also verified the Newtonian gravitational laws up to the micrometer scale.
The report in the link listed a couple of reasons why we continue to test the range of validity of this law. The most important one is with regards to the issue of dark matter/dark energy, and the possible tie-in with the anomalous acceleration of the Pioneer spacecraft. So this test isn't just "for the fun of it".
More info and exact citation to the paper can be found here. A news review from Science can be found here.