Aid. Pakistan is in crisis. No aid means disaster. It also translates into stopping military action in swat by PA since Pakistan would have no money to sustaing a war, and battle the humanitarian crisis.
In either case, Pakistan has to pay dearly. So its not wise to produce N-weapons at all..
What are US concerns and goals in the region?
If one is to assume that they want to stabilize the region (Afghanistan and Pakistan) so that the territory cannot be used as a safe haven by terrorists, then halting the Swat and FATA operations hurts the US goal just as much.
It completely isolates the GoP, Military and Pakistani populace from the US, and in fact spurs production of WMD's and delivery systems since the Pakistani establishment's suspicions about US plans and strategy in the region become firmer. It also cuts off whatever ties the US Mil. has been building with the Pak Mil. (something both Mulen and Petraeus have emphasized as critical towards establishing more trust and transparency between the two sides, leading into more cooperation).
In addition, the AID has already been linked to conditions such as greater military action against the Taliban, greater PA resources reoriented to the Western theater, expanded COIN training, working on normalization with India, access to AQ Khan.
There are already way too many conditions attached to that aid that Pakistan is strongly fighting, let alone add another condition over something as sensitive as rolling back the nuclear program.
As I said, these concerns over Pakistan's program are not new, and progress on the Khushab complex has been noted for years now - without any stoppage and is now nearing completion.
Given US goals in the region (assuming the stated ones can be taken at face value) cutting off aid at the expense of ending cooperation against the Taliban hurts US regional efforts significantly, and will do nothing to slow down the nuclear program, and in fact reduce what little cooperation and information sharing on the program (to alleviate US concerns) is currently taking place.