the only software I recall doing this was TrueCrypt?
However, it may not be very smart to do this. Law enforcement aren't dumb, they'll see the signs of your hidden partition, and you can end up in prison for up to two years for your little stunt.
Unless you're hiding something much more incriminating, cooperation is probably your best bet.
Hidden partition are definitely more dangerous. Also if these people took your device out of your view point, you need to sell the device and buy a new one before restoring anything.