Okay brief revision and I hope a better overview of current state.
Raspberry Pi will run linuxcnc but there are many problems still to overcome, I think maybe raspberry pi could be better in some respects than the beagleboneblack, but still too early to tell for certain.
linuxcnc will compile and run and drive motors simply from the base raspbian distro, but does not offer realtime unless you spend a large amount of time compiling a realtime kernel.
To compile linuxcnc from the standard raspbian distro use the following commands:
sudo apt-get install git git clone git://git.mah.priv.at/emc2-dev.git git branch --track rtos-integration-preview3 origin/rtos-integration-preview3 git checkout rtos-integration-preview3 sudo apt-get update sudo apt-get install gettext autoconf libpth-dev bc gcc g++ make git libncurses5-dev libxaw7-dev libreadline-dev tcl8.5-dev tk8.5-dev bwidget blt libgtk2.0-dev python-dev python-tk python-lxml libboost-python-dev libtk-img python-imaging-tk python-xlib python-configobj python-gnome2 python-glade2 python-numpy libgl1-mesa-swx11 libgl1-mesa-swx11-dev python-gtkglext1 python-opengl freeglut3 libglu1-mesa libglu1-mesa-dev cd emc*/src ./autogen.sh ./configure --with-threads=posix --with-platform=raspberry --enable-drivers --enable-simulator --enable-run-in-place
First realtime, there is a possibilty hard real time kernel may not be absolutely necessary if I/O is moved off to external hardware and handled properly with large prebuffering. Even the realtime kernels do not give great performance and this means a fast basethread and software step generation is not really possible.
When I define kernel as 'works' means that it compiles and runs linuxcnc and will drive I/O
Due to the poor realtime performance I/O requires special consideration as software stepgen is probably not possible unless improvements can be made in the realtime kernels.
possible ways of constant step pulses and reducing load on processor
The best options so far are a pic32 based SPI interface board called 'picnic' or using the DMA based gpio control, I do prefer the idea of the picnic as it probably protects the rpi and does voltage level conversions handles other IO types like pwm and ADC and other problems can be handled better, but unfortunatly maybe a large extra cost.
Sorry but I have erased most of this section as I have heard that rpi touchscreen will be developed by the rpi foundation for release within 6 months and will be far cheaper than any alternative, as I doubt linuxcnc will be suitable for real world use before the touchscreen is released it seems pointless considering anything other than the official touchscreen.