American Registry for Internet Numbers (ARIN) is running a trial service that gives users access to historical IP whois data — that is, it will tell you who was responsible for an IP address or block of IPs.
The service is not automated and if you want to access it you will need to submit a request via email with information about not only what you want to know, but why you are interested in accessing the information.
The service could be useful for understanding an IP block's reputation, assuming it has changed hands.
Thanks to Scott Pinzon (@spinzon) for bringing it to my attention.
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