As for the shape of a distributed exchange, I'm not so sure about that. Ideas?
A web of trust crossed with a public blockchain in which traders announce their cash-transfer capabilities (Ex. PayPal, Bank, face2face near Lat. X, Long. Y etc.). All trades are instantaneous and recorded in the blockchain at the then current price. When it's time to cash out or deposit money, you use a client program that matches someone who needs cash with someone who wants to deposit cash. The primary distinction from Bitcoin-OTC is that the persons trading cash are not also exchanging bitcoins, each has it's own time and price at which the trade took/will take place.
When a match is found, the users exchange contact credentials encrypted with each other's private key and do the out-of-blockchain money swap. If the PayPal, face 2 face etc. swap was fine, the receiver of the money credits the giver in the blockchain and each is on his way. If not credited, the giver can publicly complain and reduce the receiver's rating. The system automatically computes ratings and updates the web of trust. Highly trusted individuals could ask for a premium for their services, so they are incentivized to stay honest. The bitcoin settlements are automated and the exchange chain refers to the bitcoin blockchain by requiring traders to prove their identity using the bitcoin wallet keys.
The system would also require trusted arbiters that hold the excess cash and bitcoins needed to provide liquidity to the market. These could be selected from the highest rating individuals.
Just a very rough draft, it probably has millions of flaws that scammers can abuse. But I wouldn't call it a pipe dream. Face 2 face traders would only announce themselves in friendly jurisdictions, otherwise law enforcement can quickly single out individuals with good history and take them out based on the proof of past transactions.