All,
    I would like to takes some time to get some basic understanding about the 'nature of fiat' because there is some kind of belief that there is such a thing as 'perfect representation' when even bank accounts are not a 'perfect representation'.

   1) All modern fiat is actually a defaulted gold certificate.  They continue to trade at slowly depreciating values, but this is the true nature of fiat.
   2) USD at Mt.Gox is different than USD in Bank of America is different than USD at JP Morgan.   USD at these banks are not perfectly fungible and they trade (in a very small) range based upon the supply and demand for 'credit' at various institutions.   Ask anyone who wants to get money into or out of Mt.Gox if a $1 in Gox is really worth $1 in your hand.  Sometimes it is more, sometimes it is less.   As a result, even something as traditional as Mt.Gox, Banks, etc are not 'perfect representations' of currency.
   3) All that exists in the current banking system is dollar denominated IOUs that are exchanged against one another at prices near par.  Ultimately these IOUs have no backing and the primary difference between them and bitcoin is that a central authority can create new IOU USD with no backing.

  With that out of the way, lets step back from focusing on a 'solution', namely a p2p exchange, and grab a clearer understanding of the 'problem' where the problem is not the lack of a p2p exchange.   The problem is the market demands a money that combines the best properties of Bitcoin and fiat.   So, I am going to define our problem in terms of creating a 'better' money than bitcoin.  'Money' *is* the ultimate P2P exchange through which all other goods are bought, sold, and compared.   So, lets define why money has value.

Money is 'valued' because of its ability to move 'purchasing power' through time and space with minimal losses and risks.   
 
   Fiat money suffers from counter-party risk and inflation.  Bitcoin solves those problems but suffers from exchange risk.   Real-time exchanges like Mt.Gox can mitigate the exchange risk by offering 'instant exchanges' and 'real-time' price discovery at the expense of re-introducing counter-party risk. 

  With that said, what is the logical result of a P2P exchange in the extreme?  Every individual *is* an exchange.   You could say that the free-market is the ultimate P2P exchange!  Why do centralized exchanges exist?  Because out of the billions of people in the world only those at the 'margin' can agree on a price and these people are hard to find and trust and unlikely to be local.  Because the market demands efficiency, particularly with money, any exchange based upon 'trust' tends to concentrate the 'trust' into LARGE organizations with sufficient common trust to produce an internal exchange with sufficient liquidity and small spreads.   A P2P system that only serves to facilitate trading among a larger number of smaller 'trusted' organizations... say issuers of crypto-USD-bonds will only serve to create a large number of dollar-denominated bonds that all trade AGAINST one another with some amount of spread.  'trust' in 1000 different 'anonymous' individuals cannot be aggregated any more than 'trust' in 1000 different fiat-currencies can be aggregated into a basket while allowing each 'currency' to claim that it *is* the same as all of the others.  In effect, simply aggregating many bank notes from many 'small' banks does not cause all notes to have equal value despite the desire of the banks for this to be so.   Trustworthy banks would subsidize less trustworthy banks.

   With all of that said, we can then conclude that any IOU system must be backed by crypto-currency collateral sufficient to maintain the purchasing power of the IOU even in the face of exchange rate fluctuations between the IOU and the backing crypto-currency.   All that is required is to define the set of rules for issuing, backing, and covering IOUs that does not depend upon a trusted 3rd party to assess defaults or arbitrate collateral payouts.

   I think there currently 4 categories of exchanges:  1) those like local-bitcoins or OTC  2) those like Mt. Gox  3) those based upon decentralized issuers of 'dollar-denominated crypto-currency'  that can be traded / exchanged via systems like MarketCoin or even anonymous exchanges. and last but not least, dollar-denominated crypto-currency without any specific issuer and that has value *independent* from any individual backing it.

  Assuming you can have a dollar-denominated crypto-currency with no issuers *then* you have already achieved the ultimate P2P exchange where *everyone* is an exchange you no longer require a system to 'pair' the highest USD/BTC bidder against the lowest 'ask'.   Everyone who wants into a dollar-denominated crypto-currency is a match against everyone who wants out of it and back into paper dollars.  Therefore, it is as decentralized as possible.

  Only once you have dollar-denominated crypto-currencies does it make sense to have a decentralized trading platform that allows you to convert between currencies denominated in other units like gold, silver, EUR, or bitcoin.   

  I am going to go one-step further and claim that BTCLuke and I both require a nearly identical blockchain based trading platform.  We both require collateral with a crypto-currency.  All that remains to be solved is:
    1) how are new dollar-denominated crypto-bonds issued?
    2) how is the collateral requirement determined?
    3) how is default detected and the collateral paid out?
    4) how is the bond fulfilled?

  Both BTCLuke and I require a 3rd party escrow when trading crypto-usd-bonds for USD (except in person).  I submit that 3rd party escrow is ALWAYS required when exchanging with 'untrusted' parties when one side of the transaction involves something physical.   The only way to avoid the escrow step is if the crypto-usd-bonds were issued by a PUBLIC, trusted, and 'centralized' company.   

   I have answers to the 4 questions, and would like to see what answers BTCluke has.