It is obvious that generations of US administrations have known about the Israeli nuclear power, but they have not been not interested in exposing it publicly.
This a good solution for retaining the NPT and still allow Israel to have nukes.
E.g. not a single mainstream US paper was interested in the following story: How Israel's nuclear secret just slipped out - http://tinyurl.com/684co4
The US statement from 2008 is surprising, although it is rather hidden in a lengthy text and it is interesting that Amir Oren from Ha'aretz is exposing it in his article:
“In a rare breach of official American adherence to Israel's policy of nuclear ambiguity, the U.S. military is terming Israel "a nuclear power" on a par with Russia, China, India, Pakistan and North Korea, all of which have declared their nuclear weapon status, and ahead of "nuclear threshold powers" Japan, South Korea and Taiwan, and the "emerging" Iran.“
Ha’aretz, 8.3.2008 http://www.haaretz.com/hasen/spages/1069303.html
In the original US document in the chapter: The Proliferation of Weapons of
Mass Destruction (page 37) one can read:
"In effect, there is a growing arc of nuclear powers running from Israel in the west through an emerging Iran to Pakistan, India, and on to China, North Korea, and Russia in the east." "Joint Operating Environment", November 2008 http://www.jfcom.mil/newslink/storyarchive/2008/JOE2008.pdf
Obviously Oren expresses the opinion of the Israeli faction in the Israeli military establishment, to which also Avner Cohen belongs, that opposes the ambiguity nuclear policy, while supporting the stance that Israel but no other country in the ME should possess the nukes.