Which is exactly what I said 
Nevertheless majority of the domains used (including very large number of
spammer registered domains) are in .com/.net which is why I think this
would be usefull greylisting technique (note that its not good idea for
blacklisting unless data other then from whois is available!).
I'm going to explore this weekend what it would take to take some of
the code I have and make into the kind of library I described (this will
require addition of specialized caching database, etc) and then if others
are interested I'd like to get together with MTA and/or greylisting
spamfilter developers to finish this all up.