I was recently disabused of this notion by a friend with some expertise about the U.S. military.
He said the key to Pakistani unity and stability is its military. Despite all the factional turmoil in the country, the military stands in the background giving people a sense of national identity, stirring pride across ethnic and political divisions as a symbol of the country's historic battles for independence.
The U.S. needs to keep good relations with the Pakistani military. If the U.S. loses its alliance with the Pakistani military, it will lose all the leverage it has in the country. Invading western Pakistan, to get at al Qaeda and Baitullah Mehsud's Taliban—even though that clearly wouldn't be a hit on the Pakistan government—would put the Pakistani military on its heels.
Yes, Pakistan has a problem with militants in the Federally Administered Tribal Areas (FATA), but it's their problem. The moment the U.S. struck, there'd be no hope for an alliance with Pakistan.
This morning's headlines make me think the U.S. has found away around the problem: "Karzai Threatens to Send Forces Into Pakistan."
And isn't this cute? "NATO's International Security Assistance Force said it was not going to comment."
That's a cover. A reverse-psychology cover. They want to sound disdainful and disapproving.