We are getting too optimistic about US. When US could do no jack about A.Q. Khan, what makes you guys think US will take care of Pakistani nukes? US does everything which is in it's interests. Had the US done anything significantly or remotely close for the World's interests, being a Superpower, we would have been living in a far better world now.
US paining nerves are Israel, South Korea, Taiwan & Japan.
We have seen Iraq's situation (threat was for Israel)
Lets assume condition of Pakistan is real critical. Radical are about to take power of Pakistan in hand. Pakistan Army is supporting (covertly) Radicals.
Fear for US............ If Radical take the charge, Nukes will go into hands of them and then threat to India/ Israel (via Iran)/ South Korea (via North Korea) etc etc.
Maybe some of them will reach New york or London (Jihad).
So in this condition........ what precaution will US take? They will definitely siege all there Nuke Arsenal.