The old forum topic: http://arduino.cc/forum/index.php/topic,28744.msg212881.html#msg212881
I tried using the Sparkfun USB HOST SHIELD and the Arduino official Ethernet shield together. They came into conflict!
So, in order to not modify one of two shieldd, I did the following:
I connected the USB host shield to the Ethernet shield using some headers, I took the 10-pin header and put it out, then I made a bridge between the pin 10 and the pin 6, and finally I changed (as Felis says to do), the SS pin in Max3421e_constants.h file from 10 to 6.
Done, everything works fine without making any changes to any of the shields!