The four story, 100 ton containment dome BP lowered into place over the weekend isn't going to work because spewing oil and solidifying byproducts are keeping the apparatus from being installed properly. BP is considering a smaller dome, which would have a smaller cubic reservoir and therefore build-up less pressure.
Other possibilities being discussed include 'junk shooting' the leak site with debris that might plug the hole or installing another preventer onto the one that failed. Both these could create an even worse situation if what's left of the BOP becomes even more damaged in the process.
Meanwhile, the AP ran a total spin-story on the Administration's "aggressive" response o the spill. You can find it on the Huffington Post or by doing a search for "AP Administration Aggressive." I wont to qualify the story by linking it here. My opinion on the matter is that Obama waited too long to act.