In the past, electrical components had to be incorporated, often manually, into 3D printed objects. This was often difficult and time consuming, and meant 3D printers were often limited to producing simple, non-electrical objects. However, the 3D printing of actual circuit boards is very close, perhaps within 18 to 24 months.
To build real world circuit boards, both file standardisation, and a substrate material that works well with 3D printers are required. With these two things in place a 3D printer could select existing resistors, capacitors, and even microprocessors, and place them onto the substrate. That way, the circuit board is assembled, rather printed, although printing the actual microprocessor may not be out of the question in the future as some microprocessors are built with graphene, a typical 3D printing substrate.
In terms of file standardisation a common protocol that describes each design is required. Microsoft's .NET Gadgeteer platform, is a start but it is yet to become standard.