They never existed (I'm sure the Kurds and Iranians won't buy this)
They were destroyed (So where's the residue? That stuff doesn't just vanish, even if burned.)
They're no longer in Iraq.
Since the first two aren't really options, we have only to assume that the WMDs are elsewhere. Either the Administration doesn't know where they are, but they're still looking, or they do know where they are, but it's somewhere politically inconvenient for them. Like Syria or Iran. There's a lot of behind the scenes leverage we can use on those two if they have the goods.
Or the WMDs are sailing the Indian ocean on boats rigged to explode, and we're trying to find the people who set up the operation (Saddam) so they can call the boats back in.
That's just wild speculation, of course.
I would amend this to say "...are not [found] in Iraq..." and then add a fourth possibility:
They're better hidden than we thought.
That just means that we have to use technology to search more thoroughly for caverns, underground tunnel complexes, basements, backyards...
-PJ