(@PaulS: I agree about the place holder though I find your comment rude ("You are expected to be smart enough"??). I have also got some considerations regarding the Shield-MAC connection - see below)
(@SurferTim: Edited 1st post too and added a similar comment, thanks)
I guess it will help as a reminder if we keep adding comments to MAC such as "Do not forget to change MAC address, devices on the same address will not communicate"
A similar comment already exists in Arduino code samples (IDE v1.0.1).
// Enter a MAC address for your controller below.
// Newer Ethernet shields have a MAC address printed on a sticker on the shield
However, IMHO,
1)MAC can not reside permanently in the code, especially if you are using the same code on more than one devices
2)It is not practical to check MAC against the Shield sticker before each compilation
3)MAC should stay with the Arduino and not the Shield, because once you swap Shields you have to reprogram all affected boards!
So, I think I will write a piece of code that stores the address at EEPROM the first time you set it. That is unless there is any kind of serial number inside the Arduino that we can use to generate a MAC that is unique to the board and does not change with every reset.
Any ideas? Any unique information such as a serial number?
If we do this, we will solve this issue permanently!