In today's rather optimistic online report BP now capturing 16k barrels/day at Deepwater Horizon from The Engineer, a 150-year old publication based in the U.K., an official from Cameron International described the potential problems that may have stopped the blowout preventer from functioning.
The first problem explored by the author, "Steel casing or casing hanger could have been ejected from the well and blocked the operation of the rams."
Having part of the steel casing coming up the well was called a 'Doomsday Scenario' by an oil industry veteran during a recently televised interview:
The real doomsday scenario here is if that casing gives up, and it does come through the other strings of pipe. Remember, it is concentric pipe that holds this well together. If it comes into the formation, basically, you've got uncontrolled [oil] flow to the sea floor. And that is the doomsday scenario.