Technically possible, moderate effort, but I'm not sure how useful it would be. The majority of people get dynamic IP addresses, so over a period of a few years any given poster could have hundreds of associated IP addresses. Conversely, there's a slight, but not zero, chance that two people in close geographic proximity might at some point have shared an address. Do you have SBC DSL? I'm now living in West SJ, and there's a chance that you and I would get addresses pulled from the same pool. The odds at any given instant may be 100,000 to 1 against, but over time the odds improve.

Then there's the philosophical/policy aspect. The IP Address Game comes along regularly, usually triggered by questions about a person's identity, and for a short time a lot of people start behaving like anomalists. They come up with tiny and sometimes spurious clues that they expand into fullblown paranoia, make mistakes with the technical tools that get blown out of proportion, start viewing others with suspicion and questioning their identity too, charges of impersonation multiply...do I really want to encourage that behavior?

Any IP-lookup facility I could build couldn't provide definitive answers, so it would end up just fanning the flames. If a serious situation came up, say a court order or something literally compelling, I could come up with an ad-hoc way to get the info if I had to. I'd just as soon let the sleeping dog lie.