To short a currency, you just borrow it and then sell, in forex trading you can use leveraged trading to add to your power, and buy it back at a lower price. You can always short a currency on forex market, no matter there is bitcoin or not. And there is no such thing like you must support the currency exchange price by using bitcoin to buy your currency, you can always use USD reserve to support your currency exchange price
The only valid concern is that BTC might have devastating power over some small countries if its value gained too much, but that is another topic: What rich people will gain by destroy poor people's life???
I don't understand that either. Does anyone know why bitcoin would make such an attack more feasible? I guess maybe it's just that the IMF has plans in place to try to stop a traditional attack but hasn't spent much time thinking about bitcoin.