Hi BTCLuke,
When I initially read your post, (couple of days ago) I immediately saw the potential, the one thing that does not work however, is fiat -> BTC -> fiat, in the entire scheme you posted. (also really bonus kudos to the guy that basically already wrote the txn scheme on how to handle the p2p exchange, crypto only, as that is the way to go)
^^ We all know this can be done, and perfected as long as we are dealing with crypto currencies, with a blockchain to actually see if someone is able to spend that.
While I wanted to reply earlier, I didn't get to it being very busy. And then again I hadn't solved the fiat -> btc yet. Now I think I have gotten the clue from bitpay after their talk on the conference, and I truly believe that in combination with this p2p exchange it can work.
Forget instant transferring USD, EURO or any other fiat, it needs to digital and verifiable. (as long as there are bankers, they will not allow us to do this.)
So we gotta go outside the box... (bitpay's words)
Grab any gift card you can think of, that is automated verifiable, whether it is still valid or not.
For our example we will use Amazon and Ebay gift cards.
If I go to the store I can most likely buy an Amazon $50 Gift card for $30.
If I can check this card online whether it is still valid, then this cards code is worth $30. (despite being $50 Amazon gift card)
So I have a code, that i can check on the fly, and trade...
Now I hear you say, I don't trust Amazon. Ok fine then go Ebay.
Since there is such a huge market of gift cards, by different companies, all you have to trust is a company you already trust, that has these gift cards.
The most important part here is, to use the existing gift cards from major companies. (who people already trust, to be the intermediary between fiat and BTC)
The other thing to realize is, that crypto exchanges can do the exact same thing on this same platform.
While sharing the same platform, they can benefit from it (easier access to more crypto currencies, all of a sudden your not just exchanging bitcoin, you can exchange anything you have the client / plugin for.)
A BTC-E USD IOU, all of a sudden can be traded against a MtGox USD IOU, or MtGox Euro IOU, but at the same time vs an Amazon Gift card, BTC or LTC.
Any exchange can join, and give out IOU, depending on how much reputation they hold, their IOU's will be worth more or less. (you wanna facilitate trading of these IOU's to keep prices down)
Any Gift Card from a Major / Medium sized Company can join, as long as they are instantly verifiable as not spend. (aka, you're using your mutual trust in this company, without needing their consent)
Any Crypto Currency can join, as they are backed by a blockchain.
(And many other things that I haven't thought of, that are instantly verifiable over the internet, or have enough reputation, that people start trading their IOU's)
The combination of these 3 on a shared exchange platform, using the security of the bitcoin protocol, has the best chance of success in my opinion.
Last but not least, you could also build a sort of "localbitcoins" into this, though it would technically be nothing more than a message board, asking where to meet, with a reputation system in it. (someone looking to buy BTC, posts a message he wants to buy, the moment they meet and sell, the seller will sign the transaction as being completed, on which the system will see the transaction as completed as they see the bitcoins appear into the persoon looking to buy BTC, and anybody trying to up-rate himself, will be killed by the fees of the network upholding the exchange)
Looking forward on all input on this.