A Proof of Concept for P2P Bitcoin Exchange? Has anyone proposed or developed a proof of concept for a P2P bitcoin exchange?

Edit:

Let me add some layers here. How would trust work in a P2P exchange? How would one actually get the money they have exchanged for? Does this require a bank to be involved?
You might want to check NashX project. It's anonymous online Person-to-Person cash exchange that uses Mutually assured destruction to create Nash equilibrium for traders.

http://nashx.com/