Summary: Retarded idea. There is currently no way to implement a p2p exchange. The problem is there is no way to move fiat.
There is:
I just announced a proposal for a P2P Fiat-Bitcoin Exchange which solves the "how to get the fiat in/out" problem.
BANK RUN! - P2P Fiat-Bitcoin Exchange
https://bitcointalk.org/index.php?topic=462236