I stumbled across this as I am making a 3D-printer that is truly 100% RPi controlled. That is to say that one Raspberry Pi is used to directly control the steppers, heaters, extruder, etc, and the same Raspberry Pi is the computer that is used for making the 3D models, slicing the models into G-Code, and accessing the internet for things related to 3D-Printing (and more).

So I would say that I am making a truly 100% RPi controlled 3D-Printer, and it is almost done. I am actually using the RPi that is the controller for my printer to type this.

@Mjolinor:
Where are you going to get a Arduino, the stepper drivers, a LCD, a keypad, and the computer to program and send data to the Arduino for less than a Raspberry Pi, keyboard, monitor, some wire, a few ULN238's, and a $2 perfboard? I do not think you can, as the computer alone costs at least as much (unless you are using a RPi for the computer).