TELECOM Digest Editor noted in response to Fred Atkinson:
Since the beginning octects of the MAC address of an ethernet card identify who the manufacturer of the card is, and all ethernet switches maintain a database of what mac addresses are plugged in to what ports; if one is using manages switches it is a trivial task to locate where network cards of a particular brand are plugged into the network.
It's also quite easy on some switches to specify a range of MAC addresses that are allowed to use particular ports. So you can set that range to be only cards whose MAC address identify them as being made by3com, and lock out everything else.
Of course most network card drivers will allow you to overide the factory coded mac address on your card, which could be used by an ethernet saavy personm to overide the security methods I describe above ...