honestly, I don't see the market need for this...
...
But the easiest answer to the problem of one massive exchange isn't a complex P2P implementation of real-world transactions
The goal is not to make anything complex, but rather something simple and if possible with existing technologies.
Bitcoinx for example uses the bitcoin protocol, so everything dont need to be invented.
The system isn't the complex part - the complex part is the maintenance of the list of trusted entities, which happens on a per transaction basis, and is purely an abstract relational system that has value that differs for each and every person using the system. Every person is responsible for figuring out the level of trust they place in each and every token generated. (Think MtGox codes, BTC-E codes, etc.)
Inherent in the actual transaction of fiat for BTC is trust that the person or entity you're using for the exchange is going to do what you agree to do (perform the transaction at the agreed rate.) That's easy enough in person because you're meeting someone face to face for the transaction. It's almost as easy with a service as big as Gox because they have a history of performing as promised and they have a well understood business model. The proposed system introduces a trust token that can be traded easily, but anyone can generate their own token - meaning the list of entities CLAIMING trustworthiness is going to get very very big very very fast. The real life problem created by the system, then, is that the complexity to the average person trying to figure out who is trustworthy and who is not just got a lot more difficult. Their path of least resistance is to continue dealing with entities they already know and have a reasonable expectation of trusting, so this system will not benefit them at all; they will continue as they already do, preferring face to face transactions or transactions with large well-known commercial entities.
So if there is no practical benefit to end users, the only other benefit I can imagine would necessarily be between exchanges. The same system of trust must still be established between exchanges as the example above. No exchange will automatically trust any other exchange's trust tokens without the establishment of a trust relationship. There is, in practical terms, no difference between the trust an end user places in an emerging exchange and the trust an established exchange places in that same emerging exchange. The new exchange must still prove their trustworthiness, and each trusting entity must still make their own determination of whether or not to trust the new guy, independent of the decisions of others to trust them. Again, the path of least resistance is to continue using already established relationships with large well-known commercial entities. There are already mechanisms that provide this service, like the aforementioned MtGox codes and BTC-E codes; they are not often used because it's easier to just deal with the exchange directly in most cases, or in Bitcoin in others. Sure, they have a small place in the market, and their use is already established without the need for a separate system of trust tokens. I would not be surprised at all if the exchanges didn't already have some sort of agreements between them that stipulates transfers between them get settled at certain thresholds or at regular intervals, and they trust each other to pay as promised when called. But they don't have a need to transfer those agreements (if they exist) out to third parties - they should already have BTC/USD/whatever to be able to do those other transactions as promised.
This system is, in essence, a lot like introducing to Bitcoin the idea of floating checks made out to CASH. Someone creating a trust token is essentially saying to you "this is redeemable for Bitcoin, I promise." If you trust them, that's a perfectly valid transaction if you want to delay the receipt of BTC, but when you try to give the "check" to someone else who may not know who wrote the "check", and ask them to trust that YOU trust the person who gave you the "check", and then they spend it asking the next person to trust them to trust you to trust the person who wrote the "check", and so on until it's ultimately redeemed. All that comes back to each individual person maintaining a list of entities each person trusts. The "check", in the end, is only as good as the entity's word that it is ultimately redeemable, despite how much trust the Nth person has in the Nth-1's trustworthiness, and so we're back to large, well-known commercial entities being the best source of these trust tokens. A Cashier's check from Bank of America is going to be much more easily passable than a post-dated personal check written in a shaky hand with grease stains and a torn corner. Cash, (actual Bitcoin), is still much better, because there's no actual need for trust that it's redeemable for Bitcoin... it *is* Bitcoin.
Like I said, there may be a small niche need for this, such as the relationship between large, well-known commercial exchanges; but it's not really any benefit to the end user or to anyone wishing to become an exchange, as the trust relationships must be established just as it would need to be without this system.