Thailand has already banned BTC and China seems to be coming down hard on it. The possibility of more countries banning it (including the US and China) seems very real. What would an exchange look like that is able to get around this?
I can imagine a website in a BTC-friendly country (Denmark?) that displays an orderbook that is only updated every hour (for example). At the top of the hour, you can see the latest status of the orderbook. Then you click the order you want to transact with and you are given P2P payment instructions for a buy (PayPal, Dwolla, etc) or a deposit address for a sell. The website could maintain a reputation tracking system for each member. New members would need to built up their rep with small transactions before they are given permission for larger transactions. The time in between orderbook updates (1 hour?) would give people the chance to complete their transaction.
My feeble mind tells me this would be a great way to enable people to get fiat into BTC no matter what the governments do, but I'm pretty sure you guys are going to poke about a thousand holes in it.