I also found Dark Exchange. Which is the only example of an existing P2P exchange. I don't know if it is still in use, though it might have been moved to the tor network or i2p.
I like the idea of a decentralized exchange, but as some other have mentioned in this thread (https://bitcointalk.org/index.php?topic=45751.0) it is unlikely for such a system to gain wide acceptance with bitcoin being easy to obtain through "legal" centralized exchanges.