scswift:
If I'm not mistaken, the hardware SPI is actually pins 11-13. Pin 10 seems to be most often reccomended as the chip select pin, but as you have to set it high and low manually before you send data with the SPI lib, you could use any pin you want really, and if you want to have more than one chip using the hardware SPI bus then you'll have to if you want to select each chip in turn.
You are right for the most common case of the Arduino being the SPI Master. In that case the SS pin (D10) is left to user control so it, or any other free data pin, can be used for Slave Select. If the Arduino is acting as an SPI Slave then the SS pin (D10) is used by the hardware to disconnect the Arduino from the SPI buss when it is not being addressed.