Either they don't have them or can't get them to work--evidence, we haven't been nuked--or they're waiting for that special moment--like AQ has done in the past--waiting 2-3 years between major attacks. However, my gut tells me if they had them (working nukes), they'd of used them by now. It was one thing to wait every few years when they had a safe haven in Afghanistan, but it is another to wait while your ass is getting righteously kicked by the US.
I mean, while we're attacking them we could stumble upon their operations and put an end to it there and then. So better to use what they have now rather than wait and lose the asset. That window was in the Fall/Winter of 2001.
The "use it or lose it" argument is probably being heard a lot in al-Quida circles. If they do not "produce" it's hard for the rest of the Muslim world to take them seriously, and the ardor of their financial contributors may wane