According to a press release (12 June 2002) from the U.S. National Science Foundation (NSF), two teams of scientists have fashioned transistors from single molecules, and report their results in the 13 June 2002 issue of Nature.
At the Cornell University Center for Materials Research, Hector Abruna and colleagues wedged a molecule containing a single cobalt atom between gold electrodes. They were able, using a gate voltage, to control the transfer of electrons across the cobalt atom, demonstrating the ability to regulate electrical flow at the smallest possible scale. Hongkun Park and coworkers at Harvard University developed a transistor by inserting a different molecule containing two atoms of the metal vanadium between gold electrodes.
Additional information is available in a press release from Cornell University and an article on the Small Times website ("Researchers report they have an atom surrounded on all sides", by Peg Brickley, 12 June 2002).
And waynerad writes notes this report on the PhysicsWeb website.