Out of curiosity: If the FBI or NSA REALLY wanted to get the data on your hard drive, is Truecrypt enough to keep them out?
Fortunately for our collective sanity, if it is the case that NSA has (several times over) the moral equivalent of the "stack overflow" for cryptosystems, there is nothing we can do about it, and there's no point wanking over alternatives that might foil them.
Regarding the FBI, "almost certainly yes", assuming you use it properly (in particular, by using strong secrets). Consider that any vulnerability in the crypto stack Truecrypt uses would have far too much value to be wasted on conventional domestic law enforcement. Consider also that unlike state secrets, domestic law enforcement uses a crypto stack that is the same or strictly weaker than Truecrypt.
You need to add "assuming you are willing to go to jail" because a warrant can compel you to disclose your password, and if you refuse you'll be jailed for contempt of court.
There are some things to keep in mind though:
Fine print: As others have pointed out elsewhere in the thread, it's possible to extract encryption keys from RAM even if the computer has been (very... within several minutes) recently shut down.
More fine print: Pick a weak passphrase, and you may be shit-outta-luck. Also, brute forcing for passphrases up to a certain complexity is viable.
Tin foil hat: A conspiracy theorist may assert that the feds are far ahead of academia when it comes to cryptography, as was likely the case several decades ago (see the history of DES). I'm not sure that I believe that, because cryptography has a huge place in academia now, worldwide. It's possible, but I doubt they've broken AES.
I responded to this same comment with more details that I won't repeat, but, with respect, I'd suggest not taking seriously the crypto insights of someone who equates these two agencies.
The fact that only one organization in the US Government is likely to be able to break your disk encryption isn't a moot point, because that one organization is extremely unlikely to harass a US citizen; forget the law, the simple incentives are all wrong.
But here's the thing... I'm not paranoid in the style of "The Lone Gunmen," but this news never quite passed the sniff test. I find it difficult to believe that if the FBI was unable to crack a TrueCrypt volume that they'd let that particular cat out of the bag.
I conclude two possible motives: 1) Subterfuge to get people feeling "safe" with a method that can be broken; or 2) Demonization of strong crypto in the media as a prelude to criminalizing its use (or at least the refusal to hand over keys, as in the UK).