OK, so it seems fairly obvious that the economics of the recent oil spill can be explained using externality theory. A market creates a cost realized by a third party, and that cost is not reflected in the market price of the good. As such, the market outcome is inefficient in the sense that total net gains to society are not maximized.
My last post was an attempt to get you to think about how a single, accidental imposition of costs onto a third party might be different than a continuous release of pollution from production or consumption, and to consider how Pigouvian taxation may or may not apply in cases of a "one time" release of pollution. To get you to think about it a bit more, here's a follow-up question:
How much should responsible the responsible party pay in these situations?
For context, check out this short article from CNN Money on the clean-up costs associated with six big oil spills.
CERCLA and Superfund from the US EPA
The Oil Spill Liability Trust Fund from the US Coast Guard