Jon Chait talks about the externalities associated with oil drilling and gasoline use:
Most public attention has focused on the cost of emitting carbon into the atmosphere, but the costs of cleaning up the inevitable spills, and the military foreign policy costs of enriching petro-states, which tend to be unfriendly, and having to secure foreign oil supplies are highly significant. If all these costs were paid at the point of sale, people would switch to other energy sources.
If you were to sum up the cost of IQ losses from leaded gasoline (now gone, of course, but the effects live on), the asthma epidemic among today's kids, military protection of the Middle East, global warming, garden variety smog, plus all the more prosaic things like traffic jams and so forth, I wouldn't be surprised if the real cost of a gallon of gasoline would have to go up by three or four dollars to pay for it all. Hell, if the BP blowout ends up costing, say, $5-10 billion, which isn't an unreasonable guess at this point, that's a nickel a gallon just for that.
On another note, William Galston has a piece here speculating that the real blame for the blowout ultimately rests with Dick Cheney. I sort of hope that turns out to be true. It would restore my faith in the proper workings of the universe.