"Can a brain scan prove you're telling the truth?"

Assuming that this technology gets to the point that it is demonstrably reliable (a very big "if" IMHO), it makes sense that the first commercial applications will be proving that someone is telling the truth (as opposed to proving that someone is lying). Acquiring the fMRI data requires a cooperative subject, and a liar would not have the incentives to hold still in such a fashion that the technology requires. On the other hand, I do agree with the remark made by Joel Huizenga, "If someone is trying to convict you of some heinous crime, we think you should have every means available to defend yourself..."