Porphyrogenitus sends this link to aerial photos of Iraqi facilities for nerve gas (does that count as WMD? I'm not convinced that nerve gas is any more outrageous-ish than grenades or mines). Lets assume that nerve gas is a WMD, however, just for argument's sake.
The thing about WMD is, you can't really build it in secret. During the Cold War, our ability to spy on the Soviets using a combination of SIGINT and HUMINT was unprecendented - on a country that was astronomically huge. The USSR is so vast you could tuck two United States' into just Russia (forget about the rest of the Soviet states!). Iraq is a tiny sliver, and we have air superiority control over all of it (not just the no-fly zones).
Even civilian satellites can see our air bases. Are we blind as to what goes on in Iraq?
We can easily contain Iraq's WMD ambitions without any kind of full-scale wag-the-dog effort. The WMD argument is the weakest one, in my opinion, for invading Iraq. Since we turned the other way when Iraq used WMD against Iran, we can't invoke any kind of moral righteousness (India has a legitimate beef against our hypocrisy in that regard). So, it's clear we can prevent Saddam from building them, and it's stupid to invade just to punish him for trying.