Saddam's credibility
I suppose it is mathematically possible that the Saddam regime was completely truthful about suddenly abandoning a weapons program he had doggedly pursued for over twenty years.
However, there is a proven and documented history of the Iraqi regime lying in its teeth at every opportunity with respect to weapons programs, as well as other things. "Baghdad Bob" is a big joke now, but the fact remains that he represented official policy as far as truthfulness is concerned.
-- A timeline of deception of U.N. inspectors in Iraq up until inspectors were kicked out.
-- Other examples of deception are described in this DOD briefing. Don't forget to check out the slides.