That's one way of doing it; a large cable ISP in the Netherlands required customers to phone in when they had fried their network card. Nowadays the cable modems handed out to subscribers allow configuration of this by the end customer.
BUT: I don't think Chris and me were thinking about big bad ugly LANs with customers attached indiscriminately, though. With DSL provisioning systmes using RFC1483 bridged (do I have my buzzwords correct here?) the DHCP server can discriminate between customers based on VCI/VPI numbers instead, negating the need to look at the MAC address of the request.
When you dont have alternatives, it does seem like a possible good idea. When it costs money to add additional customers, any additional step that a customer should make gives the customer yet another reason to switch to someone that does not make them jump. Alex