1. To put pressure on President Zardari to make him do what the U.S. wants
2. To push the Pakistani army towards a coup against Zardari.
3. An attempt to steal Pakistan's nukes

Number two is now the most likely scenario. Writes Swoop:

A flurry of visits to Washington by senior Pakistani military officers is underway, to be followed on May 6th-7th by visits by Pakistan President Zardari and Afghan President Karzai. Neither man is held in high regard in Washington. Indeed, a prime reason for the military visits is that Administration officials believe some form of military rule is likely to emerge in Islamabad in the foreseeable future. "The Swat is a mess, Buner is still unsettled and tensions in Karachi between Pashtuns and Urdu camps are too high," said a US senior intelligence official, "the alternative now is either Sharif with quiet arrangements of support by the army or, just the army."

WaPo's Ignatius sees a Moment of Truth in Pakistan:

The challenge in Pakistan is eerily similar to what the Carter administration faced with Iran: how to encourage the military to take decisive action against a Muslim insurgency without destroying the country's nascent democracy.

(They had a "nascent democracy" under the Shah?)

"My biggest concern is whether [the Pakistani government] will sustain it," Mullen said. He has told his Pakistani counterpart, Gen. Ashfaq Kiyani, that "we are prepared to assist whenever they want."

Assist whenever Kiyani wants ...

Unlike the Obama administration I do not believe Kiyani will want to overthrow Zardari at all. He now can practically do whatever he wants anyway. And currently he can point to Zardari's when criticism from Washington comes up. If he would take over, the pressure from Washington, the responsibility for the economic mess and the general chaos following a coup would be his problems. Why would he want those?

Nawaz Sharif first shunned and now courted by the administration, would probably like to be president. But how does the U.S. expect to put him in charge? Zardari won elections just a few month ago - with help from Washington. He is unlike to step down and even then there would be no guarantee that Sharif would be elected. His party does not have a majority and with judge Iftikhar Chaudry reinstalled at the supreme court, there will be a watchful eye over any sleazy procedure.

Of course if some Taliban would somehow kill Zardari ...

Then those plans could succeed. But still, anyone taking over from him is unlikely to do what Washington wants. Why is the adminsitration incapable to see that?

The Pakistani elite as well as the people do see India (and the U.S.) as their big potential enemy, not some tribal mullahs in their backwoods. They fought three wars against India and they see no sign that the danger from there has receded. The Pakistani army can not just leave the eastern border and fight for U.S. interests against its own people along the Durand line. It depends on public opinion just as any politician.

If Washington wants Pakistan to pull back its silent support from the Neo-Taliban in Afghanistan, it will have to solve the India problem. A good first step would be a serious downgrade of India's presence in Afghanistan: no more consulates, no Indian roadbuilding and no Indian paramilitary police on Afghan ground. Then the problems in Kashmir will have to be solved. That may take a while but a Pakistan that will not have to fear a dual front war is much more likely to deliver support for the U.S. in Afghanistan. 

A coup will not achieve that.