We just noticed this development that was posted last July: a RepRap 3D printer was used to print a circuit board. Well, not completely – the 3D printer actually printed the etch resist, which protected the conductive bits from the acid bath. The resulting board was then cleaned up and had components mounted on it.
The 3D printing process was actually 2D: a marker pen was guided by the RepRap over the virgin board to outline the electronic traces. Two passes were required to produce a sufficient amount of resist.
There are several approaches to electronic printing, but it’s great to see another successful one.