I figured the false papers were planted as the "poison pill" to tarnish what was otherwise legitmate intel - by whatever country was acting as the go between between Iraq and Niger. In other words, the French planted it.
Both Hersh's theory, as posted by Khan, and oceanview's theory seem plausible. Both of these theories provide an explanation for the mediocre quality of the forgery, which seems designed to be just good enough to last for a while with the truth guaranteed to come out sooner or later. That's a strange kind of hoax, but these theories both explain that part of it.
So, does anybody have any thoughts on how to choose between these two possibilities?