The BP oil cap may not work due to pressure, as the rubber seal already has 600 atmospheres of pressure on it. In the case that the pressure is too great, the well could leak for years. Then we would face the considerable problem of the oil pumping out of the earth faster than our planet could take it back in. In that case, as happens with water and other liquids pumped out from under the surface, a large sinkhole would form.
Also the whole Gulf ecosystem, especially marshes such as the Everglades, are likely to be wiped out. Recently in National Geographic I read a study about oil being buried under seemingly white and glistening beaches as a result of extensive overturning of the sand to clean the beaches. That could be a danger to crabs and other fauna under the sands. Numerous bird species have already been affected, and it is anticipated that many more will become extinct if the oil well is not capped properly.