Somebody has to start developing this software as soon as possible. Nash equilibrium has always been my first choice for P2P exchanges. It's a very smart solution where no arbitraging is needed and the collateral requirement is almost insignificant.
Please, this idea needs to be implemented in code.
It already exists on nashx, localbitcoins and bitcoins.de for years.
This scheme won't work, is exploitable (buy BTC from stolen bank accounts and screw Bob), does require trusting banks and has loopholes all over the place.
All you did is describing bitcoins.de but instead of saying "they go to a website" you said "they exchange data via BitMessage" and doing multisig transactions instead of escrowing with a neutral third party.
This is good but one obstacle in the USA is how to get funds into the other person's bank account. Chase is already making it so you are not able to deposit cash into someone else's account, and other banks are likely to follow.
With this Nash equilibrium solution you can even send money by snail-mail. You don't need to trust the banking system.
Might not be the smartest idea to give your mail address to strangers if you trade a couple dozen BTC... also cash-in-mail can and will get lost - how does Bob proof to Alice he got just an envelope full of copied dollar bills and how dies Alice proof to Bob that she actually put the money in there?