To make matters much worse for the Pakis, since the mutual development of nukes, India has gone all-out to develop anti-missile missiles, with an eye to blasting anything Pakistan fires at India out of the air.
http://en.wikipedia.org/wiki/Anti-ballistic_missile#India
Having been aggressively attacked by Pakistan 3 times officially and 1 undeclared now, the Indians have no illusions about Pakistani intentions. It is a good question though, if Pakistan were even to ineffectively attack India with nuclear missiles, if India would say “to hell with them”, and sterilize their entire country, thus ending their belligerence for good.
For their part, in no way being able to match India’s anti-missile development, Pakistan has been trying to play footsie with both the US and China, the old Cold War game. Now, since the US seems to be bowing out of *that* game, they will probably run into the arms of China. But the Chinese, being no dummies, won’t stick their neck out for the Pakis, with anything remotely like a mutual defense treaty.
So the Pakis will probably turn their wrath on Afghanistan, redoubling their support for the bad guys and drug gangs.
At this point, China has more or less backed off. They've ensured that their rival, India, has a major threat to contend with on their border. They don't need to keep their hands in that pot anymore. Pakistan's as dangerous as they're going to get.
There's a number of delivery methods for nuclear weapons, especially if you're open to a sneak attack. I wouldn't pin too much hope on counter-missile systems just yet. It's a smart development, but no magic shield.
As far as sterilizing the country, Pakistan is about five nuclear weapon strikes from being reduced to a bronze age status. Take out the major population centers, and the country will disintegrate. Why spend expensive nuclear weapons to make the rubble bounce?