Michael Koplow thinks it's a failed government in Pakistan:
Anyone who knows anything about U.S. foreign and defense policy knows that the most pressing problem facing the U.S. right now is not the rise of China or the fight against al-Qaida. It is the possibility that the Pakistani government will fall and that Pakistan’s nuclear weapons will be taken over by extremists. Only slightly less worrisome is that the lax command and control structure that exists for Pakistan’s nuclear stockpile – and for those who don’t pay close attention to these things, it turns out that the Pakistani government moves its nukes in and out of traffic in barely guarded civilian vans so that we won’t steal them – will lead to a nuke being stolen or even accidentally launched. This is the reason that the U.S. keeps on propping up the Pakistani government and throwing money into a Pakistani black hole despite mountains of evidence that Pakistan is not our ally and actually works to undermine the U.S. in Afghanistan and other places.
The prospect of a mushroom cloud above New Delhi is about as unsettling as I can imagine.