UPDATED: My speculation confirmed
BP and the government have hesitated in the fitting and testing of a new cap, hyped as capable of capturing nearly all the oil flowing from the Deepwater Horizon blow-out.
This seems to support the theory that there are indications of a casing rupture or other really catastrophic problem (i.e., cement failure) with the well.
This comes in concert with news that the private and government players conducted a seismic mapping...perhaps a photographic mapping, as well...of the sea floor near the well-bore.
The press stories relating to this new cap suggest that the test process included sequential closing of valves, which would presumably increase the back-pressure inside the casing. The delay that we are seeing could be explained by a concern among the engineering minds that this pressure rise would further rupture the casing or cement down-hole. This would potentially cause a further loss of control, perhaps even below the point of entry of the relief wells now nearing completion.
As reported here, there are indications that this well could be a worse beast than is popularly understood, much less reported in the MSM.