Reading the data sheet the Arduino Ethernet Shield V1 uses digital pins 10, 11, 12, and 13 on the Uno and pins 50, 51, and 52 on the Mega. Does that mean these are the same as ICSP header, as I read it? 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. So on the Mega I can use pins 11, 12, and 13 for my projects data pins. Thanks!
AFAIKI, the ethernet shields with the ICSP connector on the bottom of the shield do not have anything connected to D11-D13. All SPI data pins are on the ICSP conector. Yes, you can use D11-D13 on a Mega for GPIO.