IP addresses and MAC addresses aren't fixed and you can change them - of course you can track down an IP address in an attempt to identify the user, but it's no more the equivalent of an SSN than a telephone number is. And I have to say with IPv6 it will be even easier to make yourself obscure because not only can you change them as with v4, they're next to impossible to commit to memory and aren't user-friendly in any way. How many IP spoof attacks bounce off your firewall every day?