When the founding fathers made the 5th amendment to the US Constitution, they probably didn't have any idea about TrueCrypt-locked hard drives. However, a ruling from the 11th Circuit Appeals Court has upheld the right of an anonymous testifier to not forcibly decrypt their data. The case relates to a Jon Doe giving evidence in exchange for immunity. The protection afforded to them under this case wouldn't extend to any other incriminating data that might be found, and as such Doe felt this could lead to violation of the fifth amendment. The validity of the prosecution's demands for the data decryption lies in what they already know, and how they knew it -- to prevent acting on hopeful hunches. The prosecutors were unable to demonstrate any knowledge of the data in question, leading the 11th Circuit to deem the request unlawful, adding that the immunity should have extended beyond just the current case.
While this is a win for US Citizens, the battle is far from over. This shows the very beginning.