1) We have no viable target for nukes if we are inclined to use them.
2) Even if we had a target, we could and probably should destroy it with conventional weapons.
3) The political fallout, in terms of world opinion, would far outweigh any gains we could hope for by the use of such weapons. Are we not the most powerful military on earth? Is it really necessary for us to hunt down cave dwelling cowards with nukes? That strikes me as the equivalent of hunting mice with my .308.
4) The 'bio' attacks on us have, so far, been amatuerish at best.
There is no 'magic number' before we start hunting down and killing the terrorists. The only thing I see as foolish is the suggestion that nukes are a reasonable weapon to use against them.
Are you in a rush to see Armageddon?