I've always thought one of the biggest weaknesses of Tor was that it did rely on a central authority. BitTorrent moved from a centralised tracker model to a decentralised DHT, so I'm quite sure it's theoretically possible. In the worst case, I think a DHT could be used as a routing layer, storing and retrieving encrypted packets in it, and achieve a similar anonymity effect... of course the latencies are going to be horrible, so it might be better to just use it as a way to setup multihop tunnels.