This is a nice post about how to create an open source CNC:
http://wyolum.com/shapeoko-desktop-cnc-mill-build/
This CNC uses an Arduino board with a CNC Shield. This CNC Shield could be replaced with this one:
http://blog.protoneer.co.nz/arduino-cnc-shield/
The software used to control the CNC is called GRBL and it was developed in C and could be flashed on Arduino board:
http://dank.bengler.no/-/page/show/5470_grbl
Unfortunately this software is not (yet) portable, this means it is optimized to run on AVR microcontroller only. Then it will not work directly on Raspi.
Other missing part is a graphical interface to use whit GRBL. I missed the GRBL Controller used to control and monitor it.
Update: Although you cannot run GRBL direclty on Raspi, you control the Arduino CNC from it:
http://zapmaker.org/raspberry-pi/running-grbl-controller-on-raspberry-pi/
Alan C. Assis 