Well, I guess the Pakistanis are just thankfull for the talibanis crossing the border into Afghanistan to do raids and not the oposite way...
The Pakistani-Afghan border has never been watertight, and there's always been raids and smuggeling going on there.
No doubth the Pakistanis lack the power to mop up taliban on the Pakistani side of the border, but do they lack the will to do it as well?
Given that the moslem fundamentalists hold a strong position within the Pakistani opinion, it could result in a fierce uprising against the present regime in Pakistan if they did it.
Bottom line is: You don't want civil war in a country with nuclear capacity!