Guess they were worried they were now on the target list, what with "recent revelations" about the ISI and the bad guys (but I repeat myself)

Financial Times:

Pakistan shuts US out of drone base
By Farhan Bokhari in Islamabad, James Lamont in New Delhi and Daniel Dombey in Washington
Pakistan has called a stop to US drone flights from a base that has launched strikes against Taliban and al-Qaeda militants on its border with Afghanistan.

In the latest sign of US-Pakistan tensions, Chaudhary Ahmed Mukhtar, defence minister, said on Wednesday that Islamabad had ended US operations at the Shamsi airbase in Baluchistan. The move comes after a surge of anti-American feeling in the wake of the killing of Osama bin Laden last month in a US raid on Abbottabad, a Pakistani hill station.

The Shamsi base, about 300km from Quetta and close to the Afghan border, has been used to launch drone strikes on militants – part of a campaign the US says has wiped out about half of al-Qaeda’s leadership.

“No US flights are taking place from Shamsi any longer. If there have to be flights from this base, it will only be Pakistani flights,” Mr Mukhtar told the Financial Times. “We have ended all US flights from the base.”

the rest here