The 3 D printer is new class of home appliance. The self replicating Rapid Protoype machines of the RepRap class may signal a parradigm shift in general robotics.
These machines are intended to be able to reproduce themselves. For example, a milling machine or lathe can be used to make all the parts for a new (better) milling maching or lathe.
Just as the first (hand made) lathe was used to produce a better lathe, a RepRap can be use to make an improved reprap.
For the time being, a RepRap cannot produce ALL the parts for a new machine. The only produce the plastic printed parts, but not the metal parrts (vitamins). Expect something to change shortly.
The RepRap class of printer is a micro processor controled plastic extrusion CNC. There are many other addative technologies that may be used, as well as subtractive technologies (milling, cutting, etc) that can be used with CNC. Expect this to see more activity in the near term.