USA can deploy long range weapons to eliminate any nation in a moments notice. Let us not assume that such an enemy is stupid enough to land itself in a spot where it can be compromised - specially if it is as experienced and powerful as USA. US may think of such a move after OIF is concluded.
Make no mistake that US would have managed to gather lot of intel on our strategic assets by now (Musharraf hinted on this). Still an operation to nuetralize our nuclear assets will be a very complex one. It will not happen unless some ghadaar in our country may decide to compromise the fate of Pakistan in exchange of exceptional incentives from US.
Terrorist attacks may also provide some justification but I am confident that we have adequate measures in place to counter possibility of such events occurring at least in our nuclear facilities.
God forbid, if US does decides to make such a move; the real issue would be to what to do of US forces which may seize control of nuclear facilities and other strategic assets inside Pakistan. We cannot nuke them, even if some warheads remain under our control. It will be a nightmare scenario of Pakistan.
Let us hope that things do not go down to such a level.