For most of the P2P overlays there are multiple serious attack possibilities (Sybil attack, routing table poisoning).

Some years ago in the university I wrote a paper about that in a seminar to summarize the possible counter meassures, but from what I remember, there were no really practical solutions. I think the most promising one is the use of a centralized certificate authority, which reintroduces some of the problems p2p wanted to solve. Does anyone know if new ideas have come up in the last years?