Despite their rampant use in popular culture, polygraph tests are highly unreliable, producing an accuracy rate barely better than chance. And the fMRI attempts are probably doomed too; we just don't currently have enough knowledge about which brain regions would "light up" when somebody was not telling the truth.
But what if? I think the applications of such a device could be wide reaching. Premeditated crime and academic cheating would probably be the first victims. Would everyone have to undergo a test once a year, to test solely for whether or not they had committed a serious crime in the past year? This would be an invasion of privacy, yes, but it would also help serve the most important role of the government--upholding the laws and protecting the life and property of its citizens.
Of course, the errors on such a device would have to be abnormally low to make it worthwhile. But if you are looking for major shifts that may occur within the next 50 years, you should consider the implications of a nearly perfect lie detector.