Especially for domains, it's important to do some validation, though in the absence of widely-deployed DNSSEC, it's hard to do automatically.

DNSSEC is not happening, blame Randy Bush and the IESG for refusing the working group consensus and imposing their own idea that cannot be deployed. An experimental protocol that increases the volume of data in the .com zone by an order of magnitude (read Gbs of data) is simply unacceptable. We do not need DNSSEC, we just need a notice in the DNS. It would be a relatively easy task to walk the .com zone and dump out a list of all the zones which contain a 'do not spam' TXT property record. This has the secondary advantage that it is not necessary to actualy consult the list, the authoritative information is in DNS.

There's a scalability problem that has to be solved, which is how to prevent a DOS-by-signing-up-too-many-addresses attack.

I do not expect that to be a problem, that would be a problem for the contractor. Limit the number of direct registrations from a particular IP address within a given time interval. It is likely to result in the cost of the system being considerably more than the cost of a couple of mid range servers and some software. This is not a new phenomena. Phill