You repeat a lie often enough, it will be accepted as truth. This seems to be the case with the infamous Niger yellow cake. Note this item from the newsletter of ex-CIA officials, AFIO:

A former top CIA official and 26-year veteran of the agency recently spoke out . Tyler Drumheller contends that President Bush was in possession of information that showed the claims he made during his 2003 State of the Union address- that Iraq had tried to obtain yellow cake uranium from Niger, were unsubstantiated. Drumheller supports his claim by recounting how the Niger story was dropped from a speech Bush made in Cincinnati in October 2002, after Director of the CIA Tenet told the President that the African story was overblown and the evidence weak.

However, Drumheller fails to mention that in the State of the Union address, President Bush credited British intelligence for the African report (not the Italian intelligence that was discredited), and that the Brits continue to stand by their evidence. [PJK/CBS 23Apr06]

In a similar vein read in Accuracy in the media: No Evidence for Prize-Winning Story . The reference is to the prison story.