The government spent $42.7 million on this study, but the results were not what was wanted, so it ignored them, spending another $220 million on anti-marijuana advertisements in 2005 and 2006. Although the report was delivered to the government in February 2005, NIDA claimed it was delivered in June 2006. The General Accountability Office, in attempting to review the study, met resistance from NIDA and the White House. News of the study and its conclusions became public in August, and the government responded that it was no longer valid because it was old data. More details in Ryan Grim's article at Slate.
The results of this study are quite similar to the results of studies of the federal DARE program, which has also been well-established to have either no measurable effect or be somewhat counter-productive. It continues because it creates the appearance of doing something to address a problem, not because it does anything actually beneficial. It's make-believe federal make-work, yet another theater performance that wastes tax dollars while providing the illusion of benefits.
Hat tip to Jack Kolb on the SKEPTIC list.
UPDATE 19 September 2006: Ed Brayton has picked up this story at his blog.