If Al Qaeda does use nukes, what would our excuse be for nuking Iran, Mecca or Damascus? The best we could do is nuke what we think is Al Qaeda's hideout...which is in Pakistan.
If we get it, we won't be able to do a damn thing that we haven't done already.
No, that's not the best we could do. We could finally take the gloves off and not have to worry about public opinion while we mopped up the streets with dead Islamists. Why, I'll bet we could even get away with putting panties on their miserable heads.