This probably won't be relevant until 2011 or so, but I'm just throwing it out there so the idea has time to simmer.

When v4 addresses are exhausted, IMO it's still going to be important to preserve v4 reachability for stuff like mail and web, even if you can't obtain or afford a v4 address. In this case I think it would be valuable to have proxies that are reachable by v4, that can be configured to turn around and talk to a v6 Linode (or an RFC1918 address on a Linode) based on domain name. You could eg do this with vhosts + mod_proxy in Apache. I suspect it would even be possible to do this in such a way that the origin's IP address is preserved, if that's possible it would be very handy.

A 'thin' proxy isn't too tough for HTTP. I'm not sure if it would be possible to do the same for SMTP, as a remote MTA trying to send multiple messages to the same IP wouldn't work out too well if it's trying to be transparent (How do you respond if 1 of many recipient MTA's has a problem? If you respond with a temporary error code, everyone else will end up getting the message twice).

This ends up being equivalent to the backup MX idea, which I myself was against... but in this case it starts to make more sense.

I suspect people wanting their own IP for a server will be able to out-bid end users for some time even after v4 runs out, but this is a situation that could become rapidly acute if we're not prepared.

Thoughts? Am I insane?