After the tragic Mumbai terror attacks, Sec. State Rice traveled to India and Pakistan to urge restraint and cooperation between the two countries--the last thing the US wants right now is a war that turns Pakistan into a failed or excessively weak state.
But Rice has also spent an incredible amount of time and effort criticizing Pakistan and urging them to "act quickly." Before the Mumbai attacks, Pakistani terror was largely a bilateral issue--between NATO and Pakistan. NATO was able to lobby Pakistan to act, but Pakistan's domestic politics made it near impossible. But now that the Mumbai attacks have enraged India, and the US can use them as a tool of threat: "Look, we have restraint, but those Indians, who knows what they'll do?"
Thus, the Pakistanis lose the highground in any negotiation--the US can control its behavior, but can play the Indians like a wild card; the Pakistanis must now crack down on terror (and take a beating for it) to avoid another war with India, which would cost them far, far more.
The sheer human tragedy of the Mumbai bombings accents this loss of moral highround--Pakistan can no longer run to the UN or other global organizations and complain that they are being bullied by the US--the US and India are trying to shore up the growing world opinion that the Mumbai attacks were due in part on Pakistani negligence. Those international law precedents on control and sovereignty start to take effect here.
Some literature (which I can't find online, of course) has been calling for parts of Pakistan to become international mandate zones--places like Pakistani Kashmir and the Northwestern Frontier Regions, where Pakistan clearly does not have administrative control. If the proper international pressure can be levied upon Pakistan, they may just have to cave and accept that certain parts of their territory be internationally controlled--incurring the world's ire by sending troops to stop them would mean collapse.
If that threat is legitimately on the table, then the (albeit weak) Pakistani leadership may have no choice but to make the hard decision and start seriously fighting, and maybe invite NATO airstrikes and support to get the job done. If the latter declaration was made, it would be a huge victory for Rice's diplomatic team.