FYI: Matt Mullenweg got back to me; I'll be meeting up with him today to discuss this further. Here is his reply to my idea, echoing our thoughts of a p2p xml-rpc solution: Me:
P.S. The other email I sent you from the Drupal-devel list has to do with coordinating an effort to build an open-source anti-spam killer that can be plugged into any OS CMS or blogware. We all love the work that's been done on Spam Karma and matches much of the work that's already gone into Drupal's spam.module. It really makes no sense for both projects to duplicate effort, so I'd like to work with you to pioneer possibly the first true multiple OS software cross-over project (that was a mouthful!). Lemme know what you think.
Matt:
Something to think more about. The next big thing coming out is a peer-to-peer semi-centralized method for sharing blacklists, it'll be over standard XML-RPC so once that's out I hope plugins can be developed for all the platforms, even wikis.