Mark Thoma has a good summary of yesterday's econ-blogosphere debate on the speculation question.

 He ends up on the question of what single cause can account for the run-up in both agricultural products and oil prices.

I am not sure he should be such an ardent Occamist here - compare these 2 graphs, of oil and wheat prices this year, and ask what sort of common explanation you might want:

If you prefer numbers, look at this ScotiaBank Commodity price index.

The Agriculture basket is in full deflation, while the storables are all still growing strongly. 

Maybe the common point is something like this. The portfolio managers piled into everything that wasn't a credit derivative between August 2007 and Feb 2008. In the case of not-very-storables, they found that the self-reinforcing loop between future prices and spot prices was weak, and so moved on. But with oil and other non-agricultural commodities, storability and market power allows the game to go on.