Some of the cable companies lock your access to the MAC address of the network card they set up the service with. To put multiple machines on the connection with a router the router must be set to spoof the MAC address of that network card. The Linksys routers I use have been able to do that for years and I'm sure any other Cable/DSL router can as well.
Other than that, most people don't even need to know there is a MAC address.