Use any pin for chip_select... but pin 10 MUST be defined as output..
According to this:http://www.seeedstudio.com/wiki/SD_Card_Shield_V3.0
MOSI (Master out slave in) is pin 11. This is what I would call output. Am I missing something?
Also where can I find a more detailed reference for the SPI.h library?
Correct the MOSI is an output as is the CLK the MISO is an input but as the other have said to get the chip to work in SPI mode you MUST have the default pin 10 set as an output. The SD card is SPI and if you use an external one the you can use any pin. aside from the MOSI,MISO and CLK pins. Remember however that you can only have ONE SS pin active at any time they are mutually exclusive.
The SPI register is basically a shift register as you toggle the CLK is shift out from the high bit onto the MOSI and shifts into the low bit the level on the MISO.