International law bestows a generous immunity (rationae personae) on sitting heads of state that shields them from virtually any prosecution imaginable. However, former heads of state receive a much more limited immunity (rationae materiae) that covers only their "official acts" while in office, and unfortunately for Bush, most countries regard torture as falling outside a government's legitimate, official acts. Such was the reasoning of the British House of Lords when concluding that former Chilean strongman Augusto Pinochet lacked immunity for ordering torture, and such will be the reasoning of several nations itching to arrest Bush and thereby establish yet another precedent to prop up the New World Order (which, ironically, Bush's daddy heralded). When you add aggressive warfare (crimes against the peace) to the torture of enemy combatants and civilians (war crimes and crimes against humanity), you have achieved the Nuremberg trifecta over which international lawyers salivate.
I relish the image of Canadian mounties squaring off against feds in reflective sunglasses, although that type of confrontation might not last very long. At a bare minimum, though, Bush will be forced to consider the consequences of his actions -- something he shows no indication of having done in his entire life.
UPDATE:
It was callous of me not to mention that a town in Vermont has begun considering whether to indict Bush and Cheney. My admiration for this turn of events is two-fold: first, because Bush's and Cheney's conduct cries out for a criminal inquiry; and second, because Vermont has once again staked a claim to its sovereign independence.