They might say "Lol, what? Bitcoins?" but (correct me if I'm wrong) breaking into someone's computer or server is considered a "serious" crime in a lot of places. On the other hand, I'm not entirely sure it would be a crime at all if the exchange openly decided they wanted to walk away with everyone's bitcoins and not give anything back. Sure, they're a representation of a currency to the ones who know about it, but there's nothing in law that even remotely mentions stealing someone's bitcoins is illegal anywhere in the world. Maybe if the bitcoin client had some sort of ToS...