The MAC address is usually locked into the hardware. Some mainframe type systems allowed changing the MAC address, which was used when you had to replace a malfunctioning board and needed to keep the same MAC address. The MAC address is not a random number; there's a code for it and changing it is not a good idea.
1) MAC addresses are, at least for most NICs, software controlled. It's very, very rare to find a MAC address "locked in to the hardware".
2) "Mainframe type systems"? You mean servers? Either way, yes, that is a legitimate (albeit rather rare) reason to change an interface's MAC address.
3) You're right that the MAC address isn't a random number, but there are quite a few legitimate reasons to change it. Provided you don't pick a colliding one, there's not really any harm that will come from changing it.
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To the OP: I've got a 2.4Ghz BlackBook, so I know what you're talking about.
First, be warned that MAC address spoofing is known to either break or no longer work the same way when Apple releases system updates. 10.5.5 and (IIRC) 10.5.4 both changed the procedure for spoofing your MAC address.
On my MacBook4,1 running 10.5.6:
The following will return with no error (provided that Airport is on), but will not change the address.
Code:
sudo ifconfig en1 lladdr 00:00:DD:C0:FF:EE
The following will return with no error (provided that Airport is on), but will only change the address if not associated with an access point:
Code:
sudo ifconfig en1 ether 00:00:DD:C0:FF:EE
There are a whole bunch more subtleties, but that should work for you. The whole MAC spoofing thing is a mess on OS X anyways. Different combinations of hardware and versions of OS X mean that the procedure is likely to differ from machine to machine -- so once you figure out how to do it, try to remember it ;-)