Yes, the Hidden Partition feature of TrueCrypt is particularly useful in this regard. Once configured and the OS set-up finished you'll have two decryption keys. One for secure use, and one for the Border Patrol. There's no way to prove how many keys can decrypt the data, nor can it be proved that more than 1 key exists. Basically, you can plausibly deny the existence of any key other than the one you give to the Border Patrol, and they can't prove different.