If they launch an attack that, hypothetically, kills 100k or so people (i.e. a nuke in a city), then I wouldn't be surprised to see some type of Muslim holocaust. I'd give Pakistan 1 day until it's a smoldering wreck.
I really think otherwise. I think this time around, with the president we have in power, we'd actually do some thinking and actually hit where it really hurts. Bombing Pakistan would only get an applause from the Indians, but it would put SERIOUS strains on the relations between the USA and China/Russia, who'd see an attack on Pakistan as a serious threat to their national security interests.
This time around, with the US being in such dire economic straits as we are now, if one of our cities gets attacked by WMD and we attack the first country that we think is harboring terrorists we can pretty much say NO ONE will help us at all. What will then take place is a HUGE military escalation against the US, which will bring the draft back in this country and as we all know, when the "ketchup gets squeezed out of the bottle, it ain't going back in the bottle" (a.k.a. war is assured).