A very serious question is why was this information - at least its gist - concealed by the US government for 35 years, through seven presidencies, while the suspicions and accusations against Israel fueled a significant amount of anti-Israel sentiment among many Americans. This concealment put Israel at a serious public relations disadvantage with the Congress and American public.
My guess is that this prolonged silence was deliberate, in order to injure Israel, and its revelation now is the Bush Administration's sop to Israel for pressuring it to make yet more concessions to the PLO.
An interesting question is whether information about these secret tapes was among the info that Jonathan Pollard conveyed to the Israelis.