If you allow anonymous, unauthenticated access to any system it will be abused. Auctions, blogs, chat, mail, phone, etc. IP addresses have never been good authenticators for applications.
This is not true if you control the IP address space and the routers around it. I mention this merely because "IP addresses have never been good authenticators" or the like is becoming a truism. For ISPs with good source filtering in place then IP addresses ARE good first level authenticators (e.g. filter lists on management ports). Note: I say FIRST level authenticators; IP addresses are obviously not suitable as the whole authentication process.
I don't know why, but I feel the need to clarify some semantics. I am sure everyone involved in this discussion already knows what I am about to say. I think the word "system" here is being abused and the context is changing. IPs are reasonable in the authentication process for network-centric items (like routers, things that make up the lowest levels of the OSI stack). Systems here means routers, or the networks they make. IPs are less reasonable the higher up the OSI stack you go. A web server may authenticate with IPs and find use in them. An application running on that web server is almost always going to find less value in that authentication since it is capable of more specific authentication (password, cookie, post rate limit, etc). This use approaches, but may not reach, the "zero" asymptote when you consider cases of applications running on private networks (VPNs, NAT networks, localhost, etc). System here means anything else, but almost never a router or the underlying network infrastructure. Yes, Geotrack has given us some more detail (of varying levels of precision/accuracy) of where IPs come from. But pretty much IP level controls (IMO) should stay at the lowest levels of the OSI stack. Ian looks to me like he was talking about routers & their neighbors. Which is a very NANOG charter way to look at things. Sean looks like he was talking about everything else (applications and things in user space). All things things NANOGers support that pays for the pretty blinky lights. I'm done. Hope that was mildly interesting or useful. Deepak