There is no known successful way of capping a well at this depth. The cap has worked at shallower depths before and they felt they had to do something besides the relief well which was put in motion immediately after the blowout. The relief well will stop the flow so they can plug it with cement.
BP will turn the relief well into a production platform at some future date, but I doubt it will be within the decade. The politics won't allow it. BP is well aware of this and there is no conspiracy.