A backdoor could possibly exist, an easy way to achieve this would be to encrypt the passphrase with a public key and store it somewhere with the data portion on the hard disk, so the passphrase could be recovered with the matching private key.
However, TrueCrypt is open source can be peer reviewed:
From the TrueCrypt FAQ
I forgot my password – is there any way ('backdoor') to recover the files from my TrueCrypt volume?
We have not implemented any 'backdoor' in TrueCrypt (and will never
implement any even if asked to do so by a government agency), because
it would defeat the purpose of the software. TrueCrypt does not allow
decryption of data without knowing the correct password or key. We
cannot recover your data because we do not know and cannot determine
the password you chose or the key you generated using TrueCrypt. The
only way to recover your files is to try to "crack" the password or
the key, but it could take thousands or millions of years (depending
on the length and quality of the password or keyfiles, on the
software/hardware performance, algorithms, and other factors). If you
find this hard to believe, consider the fact that even the FBI was not
able to decrypt a TrueCrypt volume after a year of trying.
Why is TrueCrypt open-source? What are the advantages?
As the source code for TrueCrypt is publicly available, independent
researchers can verify that the source code does not contain any
security flaw or secret 'backdoor'. If the source code were not
available, reviewers would need to reverse-engineer the executable
files. However, analyzing and understanding such reverse-engineered
code is so difficult that it is practically impossible to do
(especially when the code is as large as the TrueCrypt code).
Remark: A similar problem also affects cryptographic hardware (for
example, a self-encrypting storage device). It is very difficult to
reverse-engineer it to verify that it does not contain any security
flaw or secret 'backdoor'.
I've also found an interesting discussion on TrueCrypt being just a big Honeypot: https://groups.google.com/d/topic/alt.computer.security/LlbvEfGlnwE/discussion