I'm not exactly sure what SPI is but it needs to be enabled on my Raspberry Pi to be able to use a PiFace Digital interface - http://www.piface.org.uk/products/piface_digital/
It is enabled in Raspbian simply by selecting an option in a menu:-
http://www.piface.org.uk/guides/Install_PiFace_Software/Enabling_SPI/
If I can enable SPI I should be able to replace Raspbian with OpenWrt, which would be nice.