Weapons inspectors documented them in '98 (and before), and even the pro-appeasement Rats admitted he had them and that Iraq was therefore a national security threat. So the only real question is what happened to the them. There are three main choices:
1) Saddam destroyed them after '98 (when the inspectors left). This is laughable, of course -- a power-mad dictator willingly giving up his most potent weapons is unprecedented. And anyway, if he had done so, then why didn't he show us documented proof of their destruction as was required by resolution 1441?
2) Saddam, believing that he would be surviving this run-in with the U.S. just as he has surivived every other one, spirited the weapons to a third party (probably the Ba'athists in Syria) for safekeeping until after the war.
3) The weapons are very well-hidden within Iraq. WMD's are very easy to hide -- it's not like hiding planes, tanks, or missiles. Enough bio/chem weapons to kill countless people can be hidden in a very small space, and Iraq isn't a small country.