Approximately half of all IPv4 addresses (legacy allocations) are sort of not in the system at all. Some people argue for a marketplace that allows for trading of addresses for other considerations. As far as I can tell none of the RIRs has really bought into this notion in any way yet. Neither, if I recall correctly, did the IAB.
ARIN has sort of a wink and nod policy, which allows "directed" transfers from one entity to another, so long as the receiving entity would have qualified under the usual allocation rules. I don't think they've ever rejected one, so we don't know what would happen if they did. Presumably the two entities would have said that's nice, we're going to do it anyway and it's not our problem if you refuse to update your records. Regards, John Levine, johnl@iecc.com, Primary Perpetrator of "The Internet for Dummies", Please consider the environment before reading this e-mail. http://jl.ly