According to Haaretz:

The American administration has rejected an Israeli request for military equipment and support that would improve Israel's ability to attack Iran's nuclear facilities.

A report published last week by the Washington-based Institute for Science and International Security (ISIS) states that military strikes are unlikely to destroy Iran's centrifuge program for enriching uranium.

The Americans viewed the request, which was transmitted (and rejected) at the highest level, as a sign that Israel is in the advanced stages of preparations to attack Iran. They therefore warned Israel against attacking, saying such a strike would undermine American interests. They also demanded that Israel give them prior notice if it nevertheless decided to strike Iran.

If it is not, as "the highest level" of the Bush administration apparently believes, in "American interests" to neutralize Iran's nuclear program, could someone please explain what "American interests" are? 

As compensation for the requests it rejected, Washington offered to improve Israel's defenses against surface-to-surface missiles.

America the Superpower: Speak softly and carry a big ... umbrella.