So basically, you now have to consider the ramifications of getting your laptop's contents searched and/or completely confiscated. Do you have anything incriminating on the hard drive? Anything personal? What if you have information, pictures, driving directions to a potential terrorist target? Corporate data that you aren't supposed to let fall into the wrong hands? Believe me, border services are the wrong hands, because you have no idea what they're going to do with that data once they copy / seize it. Maybe there is some software on your PC that is, how shall we say, on 'extended evaluation'?
They don't have to prove anything, and they certainly don't need for you to provide an explanation. OK, so you decide to encrypt or password protect your system. Doesn't matter. They'll ask you to unlock the machine. Refusal is just the perfect excuse to confiscate the laptop.
Yep, this is going to make a few select people very happy on their way into the USA, isn't it?
Thanks to Ernest for