The new Ethernet shield now includes a micro SD card connector, it is MEGA compatible and an on-board reset controller. The ethernet shield connects to an Arduino board using long wire-wrap headers which extend through the shield. This keeps the pin layout intact and allows another shield to be stacked on top.
Arduino uses digital pins 10, 11, 12, and 13 (SPI) to communicate with the W5100 on the ethernet shield. These pins cannot be used for general i/o.