The default Slave Select pin is digital pin 53 for the Mega, but that does not change the SS pin for the ethernet shield. On both the Uno and Mega, the w5100 SS is digital pin 10 and the SD SS is digital pin 4.
edit: The SPI library will take care of the default SS setting in both the Uno and Mega. You only need to set digital pin 10 to OUTPUT and HIGH. The code I posted was tested on a Mega2560.