One of the key steps in the development of molecular-scale assembly is the creation of similarly-sized assembly tools that have sufficient accuracy and reproducibility to place atoms in exactly where they need to be, following a predefined plan. Researchers in New York and China have been able to create a nanoscale robot that can place molecular components accurately at atomic scale. The real breakthrough appears to be the unique error-correcting mechanisms that they implemented.
The details can be found in this paper in Nature Nanotechnology, and a nice overview is here at the NanoWerk blog.