Index buying and fear of another bad hurricane season are probably the biggest reasons. If we get through this hurricane season relatively unscathed in the Gulf, and maintain current inventory levels, energy prices could be in for a big correction as we enter the fall.

Will most likely head higher as the climate pattern changes, the big reserve buildup in China, and instability in Middle East and the Africa nations. The $100/bbl by 2010 forecast put out by Jeff Rubin and Peter Buchanan could well hit in the next 12-24 mos.

I may be repeating what's in the linked article but I'm not a WSJ subscriber.
Oil usually goes into contango when prices are low like in 1986 or the early 90's, and then the fun is in guessing how long the curve will stay sloping upward when the price goes up. There are some great strategies for profiting from a contango market and ways to lock in the forward premium; I think I have a list of them somewhere in my basement that I saved on a 5.25 floppy disk in 1993. Since I have no way of reading it I will have to forego fabulous wealth.
The main reasons for the forward prices usually being discounted are the cost and scarcity of storage and political risk. Both put a monkey wrench into arbitrage based calculations that govern financial derivatives. So as more people consume storage space to take advantage of the contango, expect the contango to end.