http://www.reddit.com/r/Bitcoin/comments/1doeih/idea_for_solving_the_exchange_problem_just_add/
(text for lazy)...
While your version of the solution gives a basic premise on what to do with the fiat, I don't see a way to replace fiat transfer functionality for anything faster than an OTC service... Clearly this escrowing of the fiat (specifically the funding of the escrow from a bank or paypal account) is not rapid enough for this system to work in real time with 100s of trades a minute like MtGox does.
And certainly no pretty real-time graphs of the trades like the MtGox API allows for, if I'm not mistaken.
I haven't solved this part yet either, but I've got some very interesting ideas on how the escrow and decentralized order book would work exactly. It's even self-promoting in that just keeping the client open and online will make the owner money automatically without lifting a finger!
Sadly, the best idea I've heard so far for a realistic solution to the fiat problem is Bitcoiner_cph' Nash Equilibrium idea above, although that was ironically suggested as an idea for use with bitcoin, not fiat.
Bitcoin doesn't ever need such a device... Escrows can be funded very quickly since the value itself moves, and it's all just one movement. Fiat, on the other hand, moves an "IOU" first followed by actual funds through a government-approved system. We have no problem coding an IOU mover, but moving the value outside of the guvmint's wires is quite a challenge.
What we really need is someone who has worked with ACH, Wiretransfers, Swift & SEPA systems, and any other banking protocols that truly understands how VALUE is moved between accounts, not just "IOUs."
Once we can duplicate the movement of a fiat's value between two peers without a centralized source like a bank in the middle, then we'll be able to hold that value (and therefore the fiat money itself) in online wallets that we trade in real-time with cryptocurrencies.