Ashley Deeks (formerly from State Department L (political-military affairs), currently a fellow at Columbia) has posted "Pakistan's Sovereignty and the Killing of Osama bin Laden." Ashley is as sharp as they come, and as readers of this blog know I think this particular topic is particularly important. Here are some highlights, but be sure to read the whole (short) piece:
[S]tates, absent consent, employ the “unwilling or unable” test to assess whether the territorial state is prepared to suppress the threat. If the territorial state is either unwilling or unable, it is reasonable for the victim state to consider its own use of force in the territorial state to be necessary and lawful (assuming the force is proportional and timely). If the territorial state is both willing and able, the victim state’s use of force would be unlawful. Thus, if the United States located a senior member of al Qaeda in Stockholm, it almost certainly would be unlawful for the United States to use force against that individual without Sweden’s consent, because there is no reason to believe that the Swedish government would be unwilling or unable to take appropriate measures against that al Qaeda member.
Although the test is easy to state, international law gives the United States (or any state in a similar position) little guidance about what the “unwilling or unable” test requires. Considerable state practice supports the existence of the test and reveals its historical roots in neutrality law, but neither states nor scholars have discussed what the standard means. . . .
...However, an important exception to the requirement that the acting state request that the territorial state act arises where the acting state has strong reasons to believe that the territorial state is colluding with the non-state actor, or where asking the territorial state to take steps to suppress the threat might lead the territorial state to tip off the non-state actor before the acting state can undertake its mission.
I think Ashley is exactly right as to the relevant law (including its relative indeterminacy), and with her further comments (read the full piece to see them) regarding the way this law maps on to the UBL operation.