Fiat has always been a problem for decentralized exchanges. Another poster here said that in order for fiat to be deposited and withdrawn from an exchange, the exchange must have a bank account somewhere that holds this fiat. Once that happens, it can no longer be decentralized, and he is correct. People recognized this in 2011 when Mt. Gox was having problems and the situation isn't any different today.
As another poster here said, it is possible to create an exchange where funds are kept in your bank account and trades occur between other people's wallets and bank accounts. Bank accounts are not really an ideal solution though since they can be used for chargeback fraud. Cash eliminates this problem but also introduces new problems such as the risk of counterfeiting and the difficulty involved with transporting it in a safe manner. You would also need an escrow and a reputation system. I believe this is similar to what LocalBitcoins is doing right now.
On the other hand, I believe altcoin exchanges can be decentralized and still retain most of the usability that they have today. It won't be easy though and I highly doubt they will fully supplant traditional centralized exchanges such as Cryptsy. Traditional centralized exchanges are far more scalable, familiar, newbie-friendly, and require no downloads or technical know-how to use. And they work pretty well as long as you don't use them as a wallet. Just deposit your coins, make your trades, and then withdraw them immediately afterwards.
You can buy a CoinoUSD asset, which represents the actual USD price, 1 CoinoUSD = 1$ (or 74.2936 NXT - latest ask order), on NXT decentralized asset exchange (as I can see same is in BitShares). And if you want to cash out, you just exchange CoinoUSD using Coinomat service to USD (see
here).
There is definetely not enough volume right now, but that's just a matter of time. SuperNET is coming
Check NXT wallets, the exchange is built into client:
SuperNET 1.2b (try Advanced mode, using SuperNET plugin you can withdraw cash to your bankcard directly from the wallet),
NXTLite (no need to download blockchain) or try
Secure Asset Exchange (SAE)
Coinomat has a fee when withdrawing USD and while coinoUSD might act as a good proxy to represent USD on the NXT AE, it isn't really a decentralized solution.
Nope. Dollars and euros can be held in physical form as paper bills and metal coins. No bank is required for that.
Yes, that would be a decentralized exchange, albeit a rather cumbersome one.
EDIT: There are also altcoins and assets that are pegged to the price of USD which could act as a proxy for USD when trading on exchanges. Examples include NuBits and bitUSD.