Check this out, I've been toying with the idea of creating a decentralized,
distributed database of abusive IP addresses to block for mail delivery.
Currently such systems exist as DNSBL's (DNS blocklists), with several DNS
servers run by an organization answering queries about which IP addresses
should be blocked. The problem is that these public systems are vulnerable
to denial of service attacks because the owner can be easily targetted.

In this proposed system PGP is key (haha) to making a decentralized network
with the same trust inherent in the central version. Check out "Distributed
HTTP server blocklist system" heading at http://www.sysdesign.ca/