I think this is showing it just got really ugly. Firing on US troops, who, if they were across the border, are invading a country? I'm not sure of the legalese, but I'm assuming even chasing terrorists over the border is off limits under international law. I wouldn't be surprised to see this get worse before it gets better. When the Pakistanis put out the 'no foreign troops' bit a week ago or so, I thought yea, lets see how long that lasts before this blows up in someone's (read, our) face.
It was working out better when they played see no evil, hear no evil on that issue. I guess with Pervez Musharaff out of power, we have to play a game of catch up on the movers and shakers in Pakistan politics.