An attorney in NY is pushing to use fMRI evidence to demonstrate that a witness is telling the truth. While there are some studies showing differences in activation between deception and truth-telling, there's broad doubt that these findings are replicable in real-life settings, where the stakes are higher and the task is less constrained. And then there's the problem of asking juries to assess this evidence.
Juries tend to be overly credulous about any evidence offered as forensic or scientific evidence. And other studies show that imaging studies generate an extra layer of overcredulousness. (On those, see Dave Munger and Jonah Lehrer.) So when an 'expert' shows a jury a bunch of brain images and says he's certain the images say a person is lying (or not), the jury will led this evidence far more weight than it deserves.