Hi wondering if anyone can help with this question.
If I store information on the EEPROM will it be erased if I re-upload my code?
Currently I have 4 Freetronics EtherMegas on my network sometimes when I update the code I forget to change the mac address.
My Idea is to store the mac on the EEPROM so I don't have this problem anymore.
I have never used the EEPROM before.
Thanks in advance
No EEPROM stored information stays in it.
You can overwrite it with AVRdude - the upload executable - but when uploading from IDE the info in EEPROM is not touched.
The library is quite straightforward - http://arduino.cc/en/Reference/EEPROMRead -
there is also a write page 
Might be a little late 
but we've added an external EEPROM to our new EtherTen/EtherMega boards specifically for storing the MAC address:
Thanks Rob that is what I needed to know.
Thanks John that is also handy to know for the future, unfortunately my boards are pre Version 3. 