It appears that Matthew Petach via NANOG <nanog@lists.nanog.org> said:
OK, I read the paper through, and they put considerably more thought into the calculation side; however, this paper explicitly calls for a centralized Pricing Authority, which is exactly what I'm advocating *against*.
There's been lots of other work like their Penny Black, and Hashcash, that let recipients decide how much work they want to see. Dwork worked for Microsoft and for a while MS tried a version of it in their mail systems. They all failed for a variety of reasons, one of the most intractable being that criminals with botnets have a lot more CPU power available than legitimate senders. Tahe usual botnet blacklist techniques don't work since the botted machines talk to the senders, not the recipients. Wikipedia has a summary of these WKBIs: https://en.wikipedia.org/wiki/Cost-based_anti-spam_systems R's, John