Joe Wilson went to Niger in late February of 2002. He reported back that there was no substance to the Niger yellowcake story:

What I Didn't Find in Africa

In February 2002, I was informed by officials at the Central Intelligence Agency that Vice President Dick Cheney's office had questions about a particular intelligence report. While I never saw the report, I was told that it referred to a memorandum of agreement that documented the sale of uranium yellowcake — a form of lightly processed ore — by Niger to Iraq in the late 1990's. The agency officials asked if I would travel to Niger to check out the story so they could provide a response to the vice president's office.

[..]

It did not take long to conclude that it was highly doubtful that any such transaction had ever taken place.

From Wilson's article, it is quite clear that his findings were known at the highest levels of the government as much as a full year before Pres. Bush's 2003 SOTU address.

The Niger Yellowcake story is a classic example of disinformation; it was widely known to be worthless well before Pres. Bush's speech.

S