Moz (above) is right that the real problem is getting through customs. Continuing his thought process, one realizes that it would be very difficult to design a product that both protects your data and is invisible to Customs. Once any product becomes even a little popular, customs will know to look for it specifically. Determining whether you have AntiCustoms Disk Proptection installed on your system will probably far easier to answer than the question of what you have encrypted, but once they determine that you have "something to hide", your life can quickly become miserable regardless of what that something is.

So, I don't yet see a lot of hope. While technically-knowledgable people can invent personal systems that customs won't find, I don't see how the majority of people could benefit from this. In fact, having a "benefit" like this only available to a few people becomes an impetus in its own right to declare it illegal.