As I said in another comment - even though obviously there is no objective way to prove it, like many other things in the justice system it's about what the judge/jury will believe. If they pull up logs that show you've used your password just before your interrogation then you can bet you'll be found guilty even if you really forgot the password.
Those are the kinds of cases that should be handled through jury nullification - too bad it's a crime to explain that right and how it should be used to jurors.
