Any encryption method is really as secure as its
encryption key is. The actual algorithm don't matter that much really (as long as it's not too trivial to reverse), and in fact, security-wise you must assume that the attacker as
full access to the whole system, including the algorithm and its implementation. The only thing you should really rely on is on the secrecy of the key, nothing more. With Bitlocker, the decryption algorithm is obviously stored within the computer, together with the data, that's OK, but you must make sure that the key (be it a password/pendrive/TPM module/whatever) is stored AWAY from that, otherwise you just have "security though obscurity", which is not real security. This is exactly what states the
Kerckhoffs's principle.
Now, another thing to note is that all full-disk encryptions are only useful before the computer has booted, before that, remote access is very limited, if possible at all, so physical access is the only way to try to break it. But another problem is that it's often considered that when an attacker can get physical access to the data he's trying to get, it's already game over, and the attacker won. He then must crack the encryption key, which may or may not be trivial, but some techniques may optimize that (dictionary attacks, selectively trying most likely passwords or just brute force if time allows).
The AES algorithm is right now considered quite solid and reliable, so both Bitlocker and TrueCrypt are not that trivial to hack, but only when used with strong passwords. Ultimately, encrypted data can ALWAYS be reversed into its original form, if the attacker knows and really wants to do so.