I work on firmware for stepper drivers, for 3D printers. The RepRap 3d printers usually use some form of Arduino to receive GCode and drive the steppers.

Raspberry Pi is a great little bit of hardware for the price, and naturally people want to use it as a PC replacement in all sorts of projects. Unfortunately, the Pi falls between two stools. It is quite underpowered compared to desktop PC, but if running Linux has too much baggage for a real-time controller. So I think it would be a struggle to get LinuxCNC running nicely on it, though I am sure someone will try ;)

If you don't run Linux, then you lose all the goodies that Linux provides, and also access to graphics acceleration via the GPU. The GPU is effectively closed as Broadcom only give specs under NDA to volume customers. However, several people are looking to port non-Linux realtime OS to the Pi.

The I/O on the Pi is quite limited too, without extra hardware. It is not possible to buy the Broadcom chip and spin your hardware either.

I am pretty sure someone will turn it into a stepper driver, the MIPS/$ for the stock Pi is unbeatable. In our printer project we are looking to see how we can use the Pi. Either as a host interface or as stepper controller.

I am still waiting for one to be delivered!