On icbit.se, you can trade futures contracts in bitcoin, to be settled December 15. Currently, the price at MtGox is 12.3$, but futures contracts at icbit are 13.7$. Essentially, you are buying bitcoins for delivery in december, why are people willing to pay 1.5$ more for that?
Of course you can use leverage to buy more bitcoins in the future, but in an efficient market both the futures price and the current price should be good indicators of what the market thinks the price should be later. Only if there are costs of storage should the futures price be above the spot market price (contango). Are bitcoiners not rational?
And why is the site not swarmed with arbitrageurs? Buy 100 BTC at an exchange, pay 1230$. Transfer a fraction of these to icbit as margin, and sell 100 BTC in december. Then no matter what happens to the bitcoin price, you will have 1370$ worth of bitcoins in mid december. If the price has gone up, you loose on the icbit account, but that is compensated by the rise in value of the rest of your coins - and reversely if price drops. An (almost *) risk-free investment returning 11% in three month (or 55% per year). Pretty good, for a risk-free investment in dollars!
Of course it is only risk-free if you are American. Us Europeans expose ourselves to fluctuations in the dollar versus euro when doing this.
(*) Only almost risk free. Your bitcoins could be hacked. Bitcoin could totally collapse. The world could end....
I'm already going this. Deal was made in July for 50 bitcoins at $11 on October 31st 2012.
This would add more interesting price fluctuations to the market if there were bigger orders of it.