No one uses IP addresses for authentication these days, right? All that went out with rhosts one would hope. Sadly it's not true and when you have an anonymising onion network you really start to understand how important IP authentication still is.
Many sites ban all Tor nodes from posting. Many IRC networks (even the `clueful' ones like freenode) ban Tor as well. This is usually caused by abuse from trolls using Tor, of course. But the only course of action that these networks have is to ban by IP address.
So, more precisely, IP addresses aren't a source of authentication as much as they are a finite resource which can be used to hit people with. Like loosing a deposit, loosing an IP address is a punishment to deter people from abuse since IP addresses are considered finite.
Now that's a pretty bad approximation and leads to people getting banned for no good reason because someone else was a troll from the same IP address. It really starts to go wrong in the face of large proxies (like AOLs), dynamic IP ranges and, of course, Tor.
OpenID is the most exciting movement in this area that I've seen for a long time. (it's a protocol which could never be written by a standards body because it's designed to work given the realities of the Internet, not despite them. For an example of the latter see IPv6).
OpenID basically lets you nominate a server as your `identity' and prove to a 3rd party that you control it. That doesn't solve anything right away because I can produce identities at will. What we need is an alternative limited resource which we can hit people with.
Hashcash uses CPU time which is a little problematic because the speed difference between someone on a dual-core, 64-bit Athlon and a mobile phone is pretty big. Mojonation used disk space - which is problematic because it's difficult to make that work in this context.
I'm suggesting that we use human time as measured by CAPTCHAs. Although the state of the art in breaking CAPTCHAs is getting pretty good, the best CAPTCHAs are still good enough. You can easily imagine a page which would take half an hour to complete and would sign an identity when done. That half an hour of time is the limited resource that you can loose.
Of course, you can hire out a sweatshop in China to solve these things, or make a distributed network of people paid in free porn but the threat model here is the Slashdot troll. And how well would your IP address blocking scheme work against the same attack?
What's the transition path? (If an idea doesn't have a transition plan that's probably because the transition will never happen; again, see IPv6.) Websites can start using this right away in the whole `single sign on' way that OpenID is designed to allow. Other services are more of a plain because specific client and server libraries need to be written along with an ssh-agent like daemon. So let's leave IRC alone for a while and see if we can get sites like Wikipedia to allow it.
(Actually, in the case of Wikipedia I'm not too hopeful. I've had a patch to improve their IP blocking pending for weeks now with no movement what so ever.)