Courtesy of the latest Wired wired/tired/expired list, I found out about IRIS, an MIT project with NSF support to build P2P networks that scale and are resiliant to attack.   The power point slide show gives the best technical detail about their methods (distributed hash tables).  An article in Dr. Dobbs gives an overview.  This is a topic I've thought about off and on for the last two years.  An index or directory of some kind is at the heart of any P2P system and make it the most vulnerable to attack.  Solve that problem and a lot of other things become easier (which is what the slide show says in 40 slides or so with a lot more detail).