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.
The DAC has no way to send data back to the atmega, so that pin will be connected to nothing. Can I use it for something else without screwing up my communications,
So how about pin 10?