From arduino http://arduino.cc/en/Main/ArduinoEthernetShield
"Arduino communicates with both the W5100 and SD card using the SPI bus (through the ICSP header). This is on digital pins 11, 12, and 13 on the Duemilanove and pins 50, 51, and 52 on the Mega. On both boards, pin 10 is used to select the W5100 and pin 4 for the SD card. These pins cannot be used for general i/o. On the Mega, the hardware SS pin, 53, is not used to select either the W5100 or the SD card, but it must be kept as an output or the SPI interface won't work."
The pin usage on the Arduino UNO would be the same as the Duemilanove. If your Arduino does not have an ATmega168 or ATmega328P processor (Leonardo, etc) you will have to look up which pins are used for SPI.