With due respects, there is a flaw in your thinking. Many ISPs give users
NATed adresses, without users really knowing or understanding what they are.
I ordered a static IP from my ISP (so I could run 6to4 :). The DSL modem
that they sent me speaks PPPoE over ATM to the ISP and DHCP to the host.
Even though I had a static IP address, the modem NATed the address and
assigned an RFC 1918 address over DHCP. And no, this isn't so it can support
multiple hosts - the modem refuses to talk to more than one Ethernet address.
Fortunately it was possible to configure the DSL modem to disable the NAT
and have the host talk PPPoE directly to the ISP. But this demonstrates to me
that ISPs will give customers NATted service even when there's absolutely no
reason to do so. And my current ISP is one of the larger ones in the US.