In IGP’s commentary regarding the letter, they claim that this letter confirms a legacy holder’s right to “own their number blocks.” While the letter is certainly an interesting read and its existence is somewhat curious, I don’t really believe this letter resolves anything.
The letter to me clearly states that the NSF general counsel believes that NSI under the NSF contract granted the organization which received the IPv4 Addresses certain rights to use those addresses. Furthermore, the letter goes on to state that NSI did not and could not unilaterally revoke those rights and that the “NSF does not believe ARIN, or for that matter any other organization, could retroactively affect property and rights distributed to you.”
In a comment to the blog post, John Curran CEO of ARIN writes “The concern has never been about ARIN unilaterally reclaiming number resources; it has been about changes to the number resources in the registry and whether such changes must comply with community policy. The letter further does not address in the least ARIN’s operation of the registry…”
My personal opinion is that the ambiguity that does exist regarding the relationship of Legacy IPv4 Address holders who has not signed a registration services agreement with an RIRs will not fully be resolved until a US federal court rules on the specific issues surrounding their status and specific “rights.” It also seems likely that US federal law or regulation could also clear up any ambiguity, but a resolution method through the court system seems more likely. Even then only further litigation can fully resolve any claims a legacy holder or ARIN claim to assert.
I encourage you to read the letter yourself to see what it does or does not say.