Sure, and that's useful but not revolutionary nor exclusive to 3D printers. You can use a milling to mill a bunch of pieces for a milling machine. You can use a PCB printer to print the PCBs for a PCB printer. A 3D printer is much, much closer to this than it is to a self-replicating machine.
reply> You can use a milling to mill a bunch of pieces for a milling machine.
replyNow that CNC mills get more affordable, people are starting to get vocal about their visions of a self-milling CNC mill. :-)