meanwhile, have a look at bitcoin.de , they dont ask for a money deposit, only a BTC deposit from the seller. So in case the buyer doesnt actually pay, nothing is lost and and the BTC can be returned to the seller; on the other side, the BTC from the seller is frozen when a transaction starts, so when a buyer did transfer the money but the seller doesnt confirm, then the exchange-owners can always check and release the frozen amount of BTC.
It is slow (because of banks), but it works.
How can they (the exchange owners) check whether the fiat money was transferred? This is the key. If the BTC seller denies receiving the money, and is lying, he gets to keep the BTC and the fiat.