Pul-lease.
Chain of events:
1. British making claim that Iraq was trying to buy nuclear material from Niger. May or may not be accurate. Our CIA is on the fence.
2. In speech Bush claims "British Intelligence reports that Iraq is trying to buy..." Notice he did not say "Iraq is trying to buy..." but "British Intelligence SAYS Iraq is trying to buy..." It is not Clinton-esque, it is a fact.
3. The veracity of the allegation comes into question. It is still not perfectly clear whether or not Iraq did or did not attempt to purchase the materials from Nigeria, just that some of the supporting documents have been declared fraudulent. One should also note that the people making the fraud accusation benefit from those papers being discounted.
In other words, as is often the case with Intelligence - the source is questionable, the accusation is questionable (but probable), the person/people countering the accusation are questionable, and the press, who don't understand Intelligence, have got it mostly wrong.