That's the long and the short - I do wonder how the US side will respond to the new evidence.
Prime Minister Sato was awarded the Nobel Peace Prize in 1974 for Japan's three non-nuclear principles. In his Nobel Lecture, he said:
I established three non-nuclear principles as a policy of the Japanese Government after deep reflection on the course Japan should take as a country which will not possess nuclear arms. This policy states that we shall not manufacture nuclear weapons, that we shall not possess them and that we shall not bring them into our country. This was later reaffirmed by a resolution of our Diet. I have no doubt that this policy will be pursued by all future governments.
He also mentions quite a lot about nuclear energy in his speech, and "its massive, and potentially destructive, power," while also expressing views on the peaceful uses of this energy.
The questions about the "secret agreement" between Nixon and Sato were first revealed by a Mainichi Shimbun journalist in 1972. Nishiyama Takichi was arrested and convicted over his coverage of the bilateral secret pact on Okinawa reversion and over the years, more information and new evidence were revealed. Nishiyama's scoop and his struggle was compared to the Pentagon Papers scandal in the US around the same time, and Mainichi ran defenses of "the people's right to know" and for the freedom of Japan's press. Sato's government was criticized for an "obsession" with secrecy.
There is no English page about Nishiyama on Wikipedia, perhaps someone will do the honours? Here is the Japanese page. I'm also wondering what happened to the secretary at the Foreign Ministry who leaked the papers to him, and who was also convicted. In 2007, Nishiyama broke his silence on the matter and spoke at the Foreign Correspondent's Club in Tokyo.
Takichi Nishiyama, "Okinawa Secret Pact Scandal"
The official report is now available at Mainichi Shimbun (in Japanese):
Clearly, Prime Minister Hatoyama and his government did the right thing to push this issue to completion. Yes, things change. The people have a right to know. But one thing has not changed - Japan's strong resolve to not possess, manufacture or bring these awful weapons into the country.
Okada's comment was reported widely as top news today in Japan:
Okada said that nuclear weapons will never enter Japan again, because the United States has made clear that its warships and warplanes have not carried nuclear arms since the end of the Cold War in 1991.
He reiterated the government's commitment to the country's non-nuclear policies, and said it would consider a stopover by a nuclear-armed US warship to be in violation of the policies.