I just had this. Not entirely sure of the root issue, but I think that RetroPie may have grabbed the SPI.
The resolution for me was to simply remove the spi module and reinsert it
sudo modprobe -r spi_bcm2708
sudo modprobe spi_bcm2708
Not sure if this gives anyone any ideas as to the issue. For reference I am running SqueezePlug (which is raspbian based) with RetroPie on top. The PifaceCAD worked straight away when I used it on another Pi with plain Raspbian, so I am sure it is related to the distribution or extras.
In the raspbmc case, I would assume that it likewise was grabbed for something else to use.
Blacklist the module you do not want to get control!
I'm trying to get pifacecad working on raspbmc.
I have installed python3-pifacecad from raspbian repositories, but when I try to launch the example script /usr/share/doc/python3-pifacecad/examples/sysinfo.py i'm getting the error "No PiFace Control and Display board detected (hardware_addr=0, bus=0, chip_select=1)"
The same command is working with the same raspberry with raspbian.
I think a kernel module is missing on raspbmc, but I dont know what modules are needed to use Piface CAD.
Any idea to get it working?