The MAC address for the NSLU2 is stored within the RedBoot loader image. It can be found at offset 0x3ffb0. If, for some reason, you need to replace RedBoot and you use an image from another NSLU2, make sure to edit these bytes.
Emacs has the hexl mode that can be used to edit hex bytes in place. Use the C-M-x key bound command to enter hex bytes.