Where is the line between unacceptable "torture" and interrogation?

Apparently they ID'd the courier that lead to bin laden's compound through KSM, who was waterboarded. Where would you draw the line between acceptable and unacceptable methods of interrogation? Is there a sliding scale between significance/imminence of a threat and the procedures used? Do you distinguish, as I do, between police actions that are meant to punish and correct (and therefore fall under the prohibition against cruel and inhuman punishment) and military actions which are geared towards elimination of a threat? I don't think every piece of garbage captured with an AK should be kneecapped, but if there is a bomb about to go off on my subway train, I want all options on the table.