Isn't it a refined product? The left has fought the building of new refineries, so maybe the ones that exist are working so much on refining oil that diesel is less available than usual. Just a total guess.
Yes, what Blogger said. To add to that is not only no new refineries, but the left and the enviro-wackos have caused the shut down of many existing refineries. The liberal-left-Daschles in the Senate blocked the President's energy policy that would have provided a wide-range of exploration, drilling [natural gas and oil] in ANWAR and other states, new refineries, refining (or converting) coal to fuel, oil shale to oil/fuel, bio-diesel from corn and other "renewables", and the list goes on.
Another problem is Mexico and South America having various problems getting the oil out...labor unrest et. al. then, there was the hurricanes that cut production from the Gulf of Mexico. Add to that the increase (explosion actually) in China's oil usage and needs for industry and the domestic side like cars and trucks for people who never had them.
Bottom line: we, the U.S.A. have a steady increase in consumption and a steady reduction (thanks to the liberals) in production and refining capacity. It has been said by even the conservative experts that the situation is not going to get better any time soon. Even with an energy bill passed and the work begins next year, it will take years to recover what the liberals and enviro-wackos with their restrictions and regulations have crippled us. Plus, the fact is that reserves are running out and alternatives like hydrogen power and others have to be developed soon for home, industry and transportation.
I'm no expert, but that is my take on the situation from all the articles and news and columns from here and other conservative web sources like National Review Online, TownHall.com and on and on.