Just to be clear, they don't need to be bullish on oil to make this trade. They aren't buying the oil and hoping the barrel price goes up - they already know what they're going to get paid for it. They are taking advantage of an unnatural spread in the price for oil on a 1 month futures contract versus a 12 month futures contract.
They execute a buy contract for, say, Dec 2008, at $42/barrel, and a sell contract for, say, Dec 2009, at $57/barrel. They have already agreed on a contract and sale price for a year from now. There is essentially no market risk. They just have to finance the cost of the purchase, since they won't get the funds from the sell for a year. They also have to physically store the oil, since the buyer won't take delivery for a year. Basically they are getting paid $15 a barrel to store the oil and float the costs of the oil for a year.
The real issue is that there is such a wide spread on the contracts, and that financing costs are currently so low.