I found the page for the SPI library:
It says the following:
On the Arduino Duemilanove and other ATmega168 / 328-based boards, the SPI bus uses pins 10 (SS), 11 (MOSI), 12 (MISO), and 13 (SCK). On the Arduino Mega, this is 50 (MISO), 51 (MOSI), 52 (SCK), and 53 (SS). Note that even if you're not using the SS pin, it must remain set as an output; otherwise, the SPI interface can be put into slave mode, rendering the library inoperative.
When they say "even if you're not using the SS pin, it must remain set as an output" do they mean you can't do anything with the pin, or is it just a warning that if you do do anything with the pin, like blink an LED, it's gotta be used as an ouput pin?
Also:
This transfer function sends and recieves a byte at the same time. What happens if after I set up my SPI communications I set pin 12 to output and use it for other things and then call this function? Will I just get garbage back? Will it toggle pin 12 back to an input?