The people here talking about shorting bitcoin to moderate the bubble have no idea what they're talking about. Shorting a bubble is about the stupidest thing you can do. Here's how that math works:
1) You open a margin account with $10,000.
2) You borrow 25 BTC from your broker and sell them on the market at $200. You hope that the price crashes to $0 and your net profit will be $5,000.
3) What actually happens is that the price doubles to $400. It would now cost you $10,000 to close your position (buy the 25 BTC back at market prices).
4) Your broker realizes this and decides -- without asking -- that it doesn't like this state of affairs and closes the position for you, wiping out your margin.
1) You open a margin account with $10,000.
2) You borrow 25 BTC from your broker and sell them on the market at $200. You hope that the price crashes to $0 and your net profit will be $5,000.
3) What actually happens is that the price doubles to $400. It would now cost you $10,000 to close your position (buy the 25 BTC back at market prices).
4) Your broker realizes this and decides -- without asking -- that it doesn't like this state of affairs and closes the position for you, wiping out your margin.
5) You are now $10,000 poorer.