What if the US government announced that it had completed a brain scanning technique that could tell within 99.9% of accuracy whether or not the subject was lying? Each subject would probably have to go through some sort of a training session first, and there would obviously be attempts to game the system. However, after proving efficacious against thousands of challengers who would have been rewarded for fooling the machine, it is determined that the scanner simply cannot not be beaten. I see three immediate uses of such a truth-detector:
1) A much lower false positive rate of convictions for criminal offenses. Law and the modern court system as we know it would not end overnight because the degree of punishment would still have to be determined. Yet it is highly likely that there would be more "guilty" pleas, which would save precious government dollars.
2) The US government might decide to give all citizens a run through the brain scanner once every 5 years and ask them whether or not they had committed any violent crimes in that period. Violent crime would plummet, although the ACLU might burn down the Supreme court in protest. Even if this technique was never employed, the mere threat could cause crime to fall, as long as the government left open the possibility.
3) Wealthy people would own a scanner for themselves and perhaps train themselves in their use. This would be very popular as a party trick for many years until it proves to be a potent marriage ender and quietly goes out of style.
Humans are so good at rationalizing and convincing themselves of semi-truths that it is possible that such a machine would never be invented. The limiting factor is first and foremost scanning technology, as I'm not sure that a test using fMRI could be made reliable enough. I think that there might be good dystopian novel here.